I'll give suggestions
Here are some idea's: Could the vent to the fuel tank be hindered to reduce fuel to the carb creating a lean condition? If you had fuel that had ethanol (which is supposed to be only 10%) in it and it was more than 10 (which has happened) percent could this cause the saw to overheat, as I think it would??? If there is enough play in the bearings this could cause an air leak, esp. if the flywheel is off balance. (I think this happened with a little Husky 36 I have and will rebuild-after 17 years of use- the flywheel "key" sheared through it out of time and the bearings had rough areas upon disassembly). I would think the least likely areas would be the base gasket area or where the engine halfs go together if that's the way it's built, since these are sealed with sealer, gaskets or both. I would check the compression release if it has one. But if it has a new or rebuilt carb and the saw run erratic and won't respond to adjustment then air is leaking. If there is a way to run the saw and squirt some carb cleaner, etc. around the areas where air could get in this would alter the rpm's and probably cause the saw to stumble or quit. My only other comment at this point is that the weakest area in a saw, in my opinion is the seals. They are subject to heat, friction, fuel, oil, and they can dry and probably go bad faster with ethanol in the gas and who knows what might be in oil mixes that wears them. Also, just because you put a saw away and it ran, it could sit a while and you go to start it and all the sudden it has an air leak. This is what happened with the 36. It ran fine. The carb had been rebuilt once and all the intake stuff gone over. I replaced the seals. I go to start it, after a few months. It didn't go on the second or third pull. Then it ran and acted crazy. I figured it was an air leak somewhere. Upon diassembly I found: fly wheel key (which is aluminum cast into the flywheel) sheared, and a slight amount of play in the crankshaft but not much but maybe enough. I pulled the whole motor apart and when I spun the bearings I could feel they had rough spots. The piston and bore wear clean, no scarring, ring was free, bottom and top end caged rollers were fine. This was after 16/17 years of 100 to 1 Amsoil, ether in the CA. gas, and use in an almond orchard. Keep investigating and learn. Just because I can, I'm getting new bearings, seals, and flywheel and I'll put it back together.