Stops running when it gets hot?

1999 Buick Regal mileage: 60,000. I work for a company with fleet of about 35 vehicles, my problem is this Buick. It stops running when it gets hot. I live in Arizona. Two weeks ago it would not start, after running it a couple of hours. This was on a Friday. That Monday when I got to it started right up. The guy drove it a week - same thing. We took the car to the dealer; they kept it for four days. They called and said they cannot find a problem. We picked up the car, four or five days ideas. Later, same thing. Hooked a snap-on mt-2500 to it and got the code 670 quad control module #4 {qcm}. Went back to the dealer the code still stored two days ago. Today they called and still cant find a problem, even with the code this time. Please throw me some