Lexus LS400 Fuel Problem

My 1993 LS400 Fuel Pump will shut off without any warning leaving me stranded. I believe the B+ voltage from the ECU is telling the Fuel Relay to shut off for some reason. When the car sits for a period of time, 1 hour or more it will then start and run just fine. Is this from bad gas? I think that bad fuel is getting hot and percolates and is vapor locking the fuel system. Premium fuel is hard to come by. Most premium fuel in Northern Illinois is nothing more than 87 or 89 octane fuel with ethanol thrown in and lots of moisture. Or do you think there is something really wrong with my fuel pump or ECU? Getting stuck out on the road without any warning can be dangerous. I had to get towed to a Lexus dealer once and they replaced the fuel pump relay which I later proved to myself that this was not the problem. The car just sat for a while at the dealer then ran fine. They can't find a problem. No error codes on the Multifunction error code display.