1992 Buick Century Throttle Position Sensor

1992 Buick Century mileage: 92,000. I am having some trouble with my car. The problem occurred suddenly. The car starts and runs fine until about 15-20 minutes into the drive, then it just shuts off. After about a 5-10 min wait it fires right back up. Then after another short drive it shuts down again. Then it repeats the process. I found that when it shuts down I lose spark. The ignition coils check out o.K. In addition, I still have power to the ignition module so I changed the module, the problem still exists. I looked at the engine-wiring diagram and found two temperatures. Sensors 1 runs the fan (which seems to cycle o.K.) The other sends a signal to the temperature gauge and to the ECM. I am guessing the problem is temperature related, so I changed the temperature sensor and the problem still exists. I got three codes from the computer. 12, which I think, is a gm computer. Test code 21, I do not know and 42, I do not know. I am pulling out my hair on this one (what little I have left) are all these sensors really needed. Cars used to run fine without them. I hope you can help. Thank you.