I have this problem with my HW97
You can normally tell when the POI has changed, my score plummets
I have a 25mm piston and comp tube, self made top hat, standard HW spring and standard spring guide and standard piston seal.
I have a custom left hand stock that I bought off somebody on another forum, I believe it is a copy of a GinB #7, it is made of beech. The inletting is a bit rough and I used to notice that my trigger was very different when the action was out of the stock compared to when it was fitted to the stock. The problem was the the stock fixing bolt (in front of the trigger) was bending the action when it was tightened. I removed the high spots in my stock and I widened the holes in the stock where the front screws fit into the action, the action now does not really touch the stock, it is pretty much free floating (the opposite of bedding the action as tried by Alex).
I think the loose front yoke allows the stock/action to move as the weather changes, because the stock is wood it will move depending on the temp and the moisture in the air. That is the only logical solution I can come up with
After free floating my action I still have the same POI shift and like everybody else it seems to be something to do with the cold. I get 5 clicks windage and 5 clicks elevation change on my rig, using a Hawke Tac 30 (I even have marks on the turrets that indicate where I need to set them to when the POI changes).
I also get higher fps when it is really cold, sometimes as much as 15fps. I am hoping that as the weather improves the changes will become less frequent and less of an issue, until autumn when things cool down again!
The fps change can be understood, it is more than likely down to the difference in the materials when the action is warm vs cold and friction etc, I can live with that but the random POI change is a serious issue if it happens half way through a comp.
PS I had the same problem when my action was in the standard stock.
If somebody ever finds a solution to the POI change please let me know!