No matter how silly the answer to a problem may sound; if everything else is ruled out, it must be the most logical avenue...
So, it's agreed then, the saw is haunted and liable to turn up on the sci-fi channel.
I say it has more than one problem. I think the saw was killed do to lean mixture. You could have a hairline crack in the head that would explain all this. It takes 100lbs to spread the crack and you can't see it. The 150 you have this ported to - fights like all get out - till' the crack opens. You only read 100 and try again.
Sounds like an old Volvo Wagon that about took me behind the shed. X-Rays would've helped here...
Cover portions of the saw with dish soap and watch for bubbles. (BTW, let's all root for a gasket) But moving from 75# to 100# after honing says he almost covered the crack in the cylind'r wall.
New cylind'r only. My final swag.