Funnily enough I'm just on with the same problem.
After fitting a short stroke kit with a hw piston seal and using the" prescribed" lube, I was getting around a 100 fps variation after around 50 shots.
The piston had the perfect resistance whereby with just the added weight of the spring it would slide slowly down the comp tube. I thought it was a breech seal problem but after replacing everything came to the conclusion that it was something to do with the piston seal and friction.Turned out the super duper"prescribed" grease ( £5 for half a teaspoon ) is garbage.
At Devon it chrono'd at 635 fps after about 20 lanes,( Maybe cos it was a tad warm eh? ) I'd set the gun up at 760 fps. Iv'e only had the gun a year so I'm only just learning the in's n out's.
Now after reading all the forums and gleaning all the information I can , I decided to do exactly the opposite.
I decided on a V-Mach washer. I didn't "size" it, I just bunged it in, and my, is it tight. I have to pries it out with the Allen key and it dint's my hand when I shove it in.
Now I haven't really started with lubes as yet but this seems to be, in my opinion, the be all and end all of spring guns.
At the moment I'm using Copper slip. WHAAAAA I hear you say. But iv'e always used it and I personally think it's one of the best all round lubes. ( It's all Iv'e got ).
After 3-4 shots at 790 fps it settles to a constant 780 fps for around 100 shots ( counted ) then slowly starts to drop off much like a PCP but not as pronounced, and over the next 100 shots ( counted ) will not lose more than 20 fps which doesn't seem to effect my POI, then I have to repeat the lubrication process much like recharging a PCP.
You can actually feel a slight but subtle difference in the gun when it requires re lubing, mainly when the cocking lever is pushed forward after cocking.
Iv'e only had to do this twice so far in the last week or so. So the V-Mach washer has only shot around 400 shots so far and isn't bedded in yet, so it's early days.
It's a bit of a ball ache, but the gun shoots snappy, smooth as silk and virtually recoil free, and more important a constant velocity.
I refitted the hw piston seal and it shot God awful compared to the really tight V-Mach seal. (As a matter of interest. ) It seemed a lot more "mechanical" if you get what I mean? And the V-Mach developed more kinetic energy.
So is a really loose, friction free seal which everyone seems to strive for the answer?
Iv'e ordered some Silicon oil and Moly slip to try and Iv'e also got some Silicon grease to try.
Meanwhile I'll just keep re greasing with copper slip every 100 counted shots, after all, on a TX it's not much more complicated than recharging a PCP.
And because of the massive resistance of the piston seal it certainly won't be as susceptible to extreme temperature shift as a loose piston! ( I hope ).
Wot wi temperature shift of guns and temperature shift of scopes, no wonder I can't bleed'n hit owt!!!!!!!!!
Because I'm without a intelligent name