Is it possible tha your fuel tank vent/vent hose is crimped or clogged? If the vent is plugged, you can develop a vacuum in the tank, especially when the engine is demanding a fairly large quantity of fuel. A clogged ent will usually let the engine run for a while, until the vacuum builds up and the fuel pump has problems overcoming the vacuum. If the engine is shut off for a while, the vacuum is gadually replaced, and the engine will run again for a while. To see if tis is the problem, wait until the next time the problem occurs, and then immediately remove the gas tank cap and try running the engine again. If it now runs good, check out the vent system.