Try rigging up a temporary fuel tank, like one for an outboard motor, and be sure it's well vented. Then take a test drive. I'm going to guess you're getting a vacuum in the tank after a few minutes of driving that's causing the engine to surge. You might have the wrong type of gas cap, or a clogged vent tube.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!