Had the same prob with my Jonsered, they are notorious for vapor locking. Try these tricks and see if helps,(it did mine) 1. Only shut it off with the choke,(over fueled condition) 2.When it is off unscrew the gas cap (lay it on it's side first) to allow it to vent. This helped mine alot. If it helps you can drill a small hole into the gas cap (1/8" or smaller) it leaks alittle but worlds better than dumping the fuel on you leg and losing the cap in the woods. Hope it helps ya out. Had an old cutter I worked with in the Big Horns tell me to try these tricks.

 

GB, it sounds like your tank vent is blocked. It should have a vent on the side with a hose coming up around where the Carb is (at least thats what the Husky's do). You shouldn't need to drill a hole in the cap. The old saws ran better with one, but often thats because the old tanks vent was in the cap, so if you just drilled the vent hole our, it would unplug the varnish that built up in it.

 

DM, you just need one of these: