R9 good for 20 shots, give or take

[Tom1340]

8 months ago, I posted that I could place 15 shots in a row into a dime (22 cal R9 @25 yards), but after that, my shooting fell off.  Since then, I have been through the gun several times... new ARH springs, new vortex seal, polishing & lubing wedge port, multiple scopes (my apologies to Hawke), etc,,,etc.  Now, 8 months later, I've fine tuned my analysis.  My accuracy & groups (these are not the same thing) are very good for 20 shots, then the groups start spreading a bit, and the group "center" moves down over the next 10 to 20 shots.  They seem to stabilize about 1/2 - 3/4 inch below the initial group.  I usually wear myself out about then and stop shooting, so I'm not really sure how stable the new poi is.    In 24 hrs, when I shoot again, the process repeats itself.
This really had me baffled.  But Jim of UK (in another posting) recently attributed this kind of behavior the heat buildup on the full face piston seal used by most springers, both break barrels and fixed barrels. 
So have I found the reason for my guns performance?  If so, I would expect most of you guys and gals have the same issue.  Well...?