Backfiring, sputtering, and stall

When it starts acting up again leave the gas cap loose and try it again, that'll eliminate the vent as the problem. If it it still does it you'll need to plumb in a fuel pressure guage, bringing it out so you can see it while driving, as close to the carb as possible to eliminate low pressure as being the problem. Have you changed the fuel filters and there also maybe one right at the carb where the fuel line connects.