Interesting case indeed.The fact that it gets more consistent after warm up is quite a puzzle, sounds like a diesel engine . The type of grease used could also be a factor (more fluid when warmer or stirred). With a bit of structured analysis you could eliminate the factors involved (human, pellet, mechanism, grease) step by step. Strap the gun down and take the shots through chrono. Round two (after cooling down) Inspect the pellets & clean barrel before taking the shots. Still no consistency from the start, open it up clean the seal, relube, degrease inside of the tube & smooth up with wire wool, check for inconsistencies. Most likely suspect I can think of is indeed the warming of the piston seal and its interaction with the grease & inconsistencies in the tube. Maybe in combination it behaves in this fashion.