RKG

Hello everyone. I've been doing some work on my specna arms M4. I've put a new hop rubber, tightbore barrel, new piston head and corrected the AOE. The compressions on the piston is really good now whereas before it was non existent. It's shooting at about 300fps with 0.25g BBs with a sp100 spring. The only issue I seem to have is that every few shots the FPS seems to jump to between 340 and 320, which is obviously over the limit for a 0.25. Does anyone know what the issue could be?