The best way to rule out the gas as an issue is to rig up an auxilliary tank of gas and run it on that temporarily to see if the problem persists. Something like this:
http://willardssmallengines.com/shop...-gas-tank.html would work. If you have a generator, roto-tiller, or some other small 4-stroke, engine powered device, with an easy to remove fuel tank, borrow it for the test.