1991 Pontiac Transport Engine stall

1991 Pontiac Transport 3.1 L Engine with about 120,000 miles. Automatic transmission. Engine is original, never been opened. Only engine repairs have been water pump, power steering pump, alternator,
and the sensor at the throttle base that controls idle. About 5,000 miles ago, replaced spark plugs and plug wires. The fuel filter was replaced about the same time and due to all the metal particles in the filter, the fuel pump was replaced. About the same time, there is a relay in the transmission was replaced because once the engine was warm, engine would stall when the car slowed down such as at a stop sign. It took several restarts and a lot of engine reving to keep it going. The relay fixed that. Now I can drive it a short distance, and stop at a light and the engine dies as if you have turned the key off. Once it sits for a few hours, it will start right up and run as if nothing has happened. I doesn't do it everytime you use it but since you never know when it will happen, I don't want to drive it until I can resolve the problem. The service engine light DOES NOT come on. I don't have any diagnostic devices but by shorting the contacts in the diagnostic port, the service engine light flashes a code 12, engine not running code and nothing else. The outside temperature doesn't seem to affect the situation. It has happen on both warm and cold days. The last time it was very cold and in the middle of rush traffic. Help please if you can offer any suggestions. Thanks.