Probably the easiest way out is finding your local Stihl dealer with a decent mechanic. Let them have at it. It's what they get paid to do. One of mine had similar problems. They dealer's mechanic went through it, replaced the carb, tuned and cleaned it for $46. They had the carb in stock and I got it back the next day.
We don't cut much firewood, but the saws get severely abused in marine construction and hurricane cleanup. Stihl's will still run after being on the bottom of Mobile Bay. They just need a little luv from time to time.
