Misfire at cruise, popping/afterfire at 3000RPM

DO NOT TAP the valves with the piston parked at TDC!
Ok, at the top of the compression stroke you can, but be careful that you do not tap a valve deep enough to smash into a piston.....
I'm not sure yet that OP knows where TDC compression is. Which is why I carefully worded it the way I did.