Check the metal lower case half where it meets the plastic case for warping I went through a 350 that I got as a basket case, P/C were fine but had some kind of running issue. I replaced the bearings, rings, etc. When it was done it had an air leak that I just could not find, so it went on the shelf for 2 years...A couple months ago while Weimedog's gtg he mentioned tinkering on 350 saws and that was an issue on several he had gone through.
Disassembled my 350 and sure enough, it was warped. You can tell by the thickness of the gasket maker between the halves, It was squeezed thin near the bolts and a bit thicker between them (that's where it was leaking) Also there will be a bunch of oily residue down there as well.
Blocked it true with some 400 grit paper and reassembled it. Darn saw runs great now! CSW