this is just my 2 cents on 2 strokes, from what i've learned, and from what experienced people have told me;
i use high test gas; it goes bad slower; and the dealers have all told me over 30 days, pour it in the truck.
i check the plug early on; i make sure i have the right mix ratio(just a tad rich). i keep at least one fresh new plug around. i clean the whole thing, air filter included, off with a brush after i'm done, and loosen the chain. i run the saw dry when i use it; never store it with gas in it, as it builds up gum. i use 'seafoam' in my gas-the stihl dealer says it saves a lot of trips to their shop.
pull the muffler once a year, and burn the thing with a propane torch...it'll smoke like crazy; clean the port with apopsicle stick (don't leave splinters in there)
changing fuel and bar oil filters once in awhile saves frustration. flushing both tanks out some times is agood idea; they get little chips in them..can't help it with all that junk floating around. if it's really cold, a little kerosene added to the bar oil is good (just a little).
bailey's is worth keeping onyour favorites; they are full of good advice, and good parts. they been good to me; i change out the stihl bar and buy a oregon that i can grease the sprocket tip on after the stihl bar goes wonky and gets burrs. i buy the 'full up' chisel chain....the 'safety chain' makes dust, the other stuff makes chips, but you got to watch your self. flip the bar whenever you change the chain; i get two chains; i sharpen the old one before i take it off, then flip the bar for even wear, and use the stored chain(it's already sharp and ready to go).
this saw you got sounds like it may need gentle rehabilitation....if you got a manual, go through it replacing anything that looks like it's wore out; if not, a good dealer or repair guy would be a great asset....i have a little two stroke generator someone gave me-i had to play with the air jet-it tried to run just like you're describing...maybe you got a little piece of something in the jet channel....
rebb