You might check the rubber boot between the carb and the engine. They have a tendency to come loose or crack. If a major leak, it won't run. If it is only a small crack/leak, then it won't idle well, it will require extra choking to get started, and then run ####ty after started. That or it will just scream when you finally get it going, and you can burn up your cylinder with a lean burn.
If you attempt to adjust the carb to richen the mixture (not practical on the MS 200t, as Stihl tries to make it un-adjustable) without having detected the real problem, then you get unpredictable results: sometimes it runs great (right after you adjusted it), and then it runs badly when you are up in the tree.
A similar situation on a well used saw is a crankcase leak on the crankshaft seals. A good way to detect that problem is to get it running, then hold the saw while idling in every position possible (up, down, left, right, upside down). If you find that it dies while idling every time you turn the saw on it's left side (or any other direction), and runs the rest of the positions, you probably need to break it down and replace the crankshaft seals. This is not a simple job, and should be done by a skilled mechanic.
dhuffnmu: reed in the carb ? Are you perhaps referring to the diaphragms ?
Last time I did the carb in a 020, I didn't notice any reeds.
