you have ruled out one of the variables already. If the gasoline in the GGL is the same as the gas/oil mix, then its the methanol. Where do you buy your methanol from? Another thing to consider is, even though your fuel jugs are sealed, if the air is fairly humid, the fuel doesnt have to be open long to go bad. The local club "guru" and I have had a few discussions about this. Some of our members dont use fuel can fittings. They just take the cap off every time they fuel up. In manitoba, its fairly humid, and these certain people always seem to have engine troubles of some sort. We keep telling them that its b/c they are always taking the cap off the fuel jug, but I dont think they belive us.
So basically what im saying is, Try all new fuel. I have thought about using fuel pumps to pump the fuel into the jug. Have marks on the jug for the ratios. I think this will help quite a bit.