Thanks for your response.
The mechanic brought up the possibility of sugar in the gas as he had seen it once before. We always lock the car when we leave it anywhere in public and when the car is locked, the door to the fuel filler cap is also locked. Thus, we don't feel sugar in the gas tank is the problem, but we can't rule it out 100%.
Could "bad" gas do this? If it was sugar, do you have any idea how to confirm it and, more importantly, how to purge it from the system?