I feel for you fella I really do. This is virtually exactly like the problem I've had over Winter.
I hate posting about it as no one wants to hear excuses ... me included but this is driving me crazy.
My case ... HW77k with V Mach kit fitted ... in a Paul Wilson style FT stock ... but I'm shooting HFT.
I could go down the range and zero and practice at 35 yards and put AA Field 4.52 8.4gr on top of each other on a decent still day. I've had c-t-c of 6mm. At my max of 45 yards it's done 12mm. Then suddenly the poi will move ... always vertically ... by a good inch. It's over 20 clicks on my scop but that is 1/8th and at 9.5x.
Once I've rezero'd the poi's are all the same for all my aim points. The whole thing just moves up or down. When I've turned up at shoots and the zero has held ... and then held for the 30 shots through the course I've shot several over 55 / 60 scores ( I'm not saying I'd get that all the time ... these have been on days with sensible wind ). I've also had courses where I'm 22 out of 30 targets through the course and I've scored 43 out of 44 at that point ( so missed 1 target ) ... and then just cant hit a target ( and I mean a 40mm kill at 35 yards ) on the next 8 and I've ended up with say 52 / 60 ( 7 misses out of the last 8 ). Go back to zero range and check and yup ... the zero has moved 25mm or more vertically. I've scored 58 / 60 one day and the very next shoot in very mild wind I've scored 42 / 60 on a similar course and virtually all my misses are vertical by quite some way.
I thought it was the scope ... so changed that. Mounts ... changed that. Pellets ... changed them but went back to AA Field. I then noticed that the profiles of the AA Fields changed depending on batch and I was convinced it was that. So I made sure I bought the same batch and then measured them in a Bic tube. Still this vetical leap occurred.
I was then convinced that it just had to be me. So I made sure I was holding the rifle exactly the same and releasing the shots exactly the same. I will put pellet after pellet in a tight group at 35 yards ... and then just suddenly ... poi has jumped over an inch ... and then I'm putting pellet after pellet in that new group.
Just to totally fry my brain ... when the temps dropped to freezing I noticed that the power went up ... from about 780fps to 815fps. So I stripped the gun and tuned it down to 765 warm and 800 freezing. However the vertical jump kept happening, even if I took the gun to the woods and left it in the freezing cold for half an hour and then zero'd. I would zero in the cold and then the gun would shoot accurately but then it seemed as if after a number of pellets ... say 20 or so ... the poi would jump back down ... so I was convinced that the gun was getting cold and the fps going up ( and the poi going up ) and then after so many shots the gun had 'warmed up ' internally and the fps coming back down and the poi coming back down. I must admit that I've not taken the chrono to the range and tested this out on the chrono. To confuse this ... I took the gun to the range a few days after I'd shot the World's and had a disaster on the 2nd day. The temps were much warmer in the woods back home that day ... say 8 degrees instead of freezing. My first 5 pellets went through the bull at 35 yards and all inside 10mm. So I presumed it may have been the temp at the World's ( ie the freezing temps in the early start on day 2 had sent the gun high and the poi had gone high ... all my misses were high ... even at 40 yards plus where the poi should be below the cross hairs, I was missing above the cross hairs. I was too cold to mess about on the zero range that day and virtually went straight on the course. On day 1 I'd put about 60 pellets through the gun on the zero range waiting to shoot the afternoon. So wondered if the gun had 'warmed up' as I'd seen at home, as my misses on day 1 were all sideways due to mis reading wind and I was happy with a 52/60. On day 2 ... straight on the course ... I dropped 16 points in the first half of the course and just 4 on the second half and I shot all the 2nd half kneeling. So I wondered if the gun had 'warmed up ' after those first 15 pellets and had then returned to zero ? ). However, after shooting for a while in the woods at 8 degrees the damn jump came again and my pellets started hitting over an inch high. So was it the cold ... or is the cold just adding another variable ... ie there is the original jump and also something happening with the temp? Brilliant.
I'm not the greatest shot but I'm not bad with a springer but I feel I've wasted most of winter driving around the Midlands not knowing whether I'm missing because of me or whether the gun has thrown another wobbler. I'm seriously considering giving up on the springer and using one of my pcp's.
So what am I going to try now? Well, like you, I've thought that this may be an issue with the action moving in the stock ( like you I've been checking all bolts all winter ... I even thought at one point it may be the Venom silencer/end catch or the cocking aid on the underlever ... so I got them off ... cleaned them up and loctited them all back on with new grub screws ). When I take the action out of the stock the large hole where the front trigger pin lug sits ( the lug that screws in to the end block through the main tube and the anti bear trap sits under ) is not perfectly round. It looks quite rough. So I'm thinking it may be slight movement there ( I haven't tried hitting with wood like you yet ). So Paul James advised me to wrap the lug in clingfilm and put some car filler in the stock hole and push the lug in, so that the fit there will be tight. So I will try that. My other option to see if it is this stock is to just use one of my other HW77 stocks for a while and see if it goes away.
I'll tell you what is in the back of my mind ( I'm sorry this has turned into a mammoth post but this seriously has fried my brain this Winter ) ... when I stripped the gun to tweak the power down ... the rear guide on this V-Mach kit is black plastic ( delrin ? ) with a metal rear end stop. The guide is tight when I push it in the spring. When I used to tune 77's myself and used to get a local guy to make the guides I had them made a 'slide' fit but not tight. I just can't help thinking this is too tight and that the spring may be catching on it at times and then at other times it's fine. I've not actually taken the chrono to the woods and shot a few shots and noted the poi and the fps ... and then tested the fps again as soon as the poi moves. I should do that to see if the change in poi is related to change in fps ... but you have said that yours doesn't seem related and like yours ... the poi curve through the range stays the same ... the whole lot just moves vertically.
I've not really helped you there fella I'm afraid ... but at least you know that you are not alone.