Usuallly, if an engine's been running well, and then the idle starts to become problematic after having been good, the usual culprit is an eroding cylinder/piston fit. When the cylinder/piston fit starts to go, the engine will still run OK at high throttle levels, but will tend to have idle problems. Acceleration is the next thing to become problematic.

We'd believe that if you replaced the piston/cylinder assembly, your engine will get its idle back.