Ok thanks, however I discovered that fuel heat was not my problem. I tried the same flight again with the fuel heat on and the same thing happened.

I think my fuel config might have been the problem. The center tank had 3000 pounds and the wing tanks were full at takeoff. Once I reached cruise, i think the center tank ran out of fuel and the engines died. But the crossfeeds were all open so I guess I don't understand why the engines died. The engines wouldn't start up again until I turned the center pumps off.