Well, you should make sure the vent in the cap is clean anyway (spray with some carb cleaner) but it still sounds like a blocked carb. The gas cap being off is probably just helping in the sense that it's doing the opposite of creating a vacuum, whatever that's referred to as...increasing gasoline flow similar to the way the carb on a pipe works with air, if that's something you are familiar with. But if it is still stalling under full load, that's just a band aid.