When I brought my 55 2-ton home, I cleaned the fuel system out and put new gas in it. Everything was fine for a week or two then the rubber line coming out of the tank swelled up inside and would do the same thing. If I let it set for a bit, it would fire right back up, then die after a while, you may have something similar going on, especially if you are running dual carbs and pulling more fuel flow than before.
