1996 Chevy Corsica service engine soon light

1996 Chevy Corsica mileage: 81,000. I swear my Corsica is haunted. This isn't the first incident, but it's the most recent. Perhaps you have a clue as to what's going on. Monday morning, the "service engine soon" light comes on, within 15 seconds of starting the car. At first, there's no noticeable degradation in performance. After it warms up, it seems to be running a bit rough. Also seems to be getting just a bit warmer than normal. Drove it to work this way, but decided when I got there to take it to a shop. I took it to a local shop near where I work. Their computer scanner indicated that the car needed an oxygen sensor and an ignition module. Not knowing what an ignition module was, I remarked that the car started and ran, so I couldn't understand how an ignition module would be the problem. I asked them to replace the O2 sensor and change the oil. When they were done, the car behaved exactly the same, the "service engine soon" light still came on, and it still ran rough. Not wanting to be further taken advantage of at that shop. I took the car to a different shop near where I live, the next day (perhaps that was my biggest mistake). They confirmed that the codes indicated that an ignition module was required. After asking them to make sure it wasn't something simple like a spark plug wire, and after waiting an entire day for them to look at the spark plug wires, I told them to replace the module. They did, and the car STILL did the same thing: service engine soon light is on, runs rough, and the computer codes STILL indicate it needed an ignition module. I had them reinstall the original module and RAN away from their shop as fast as I could. I should have known better than to take the car to the same shop anyway, but I never imagined it would really be a tough nut to crack. So, my question is, have you guys ever heard of a similar situation? That is, a computer indicates two parts need replacing, you replace them, and the problem still exists. The code for the ignition module still returns even after being cleared multiple times (and the battery being unplugged). The service engine soon light still comes on, and it still runs rough. Spookier still, the light doesn't always stay on. This evening after work, my wife took the car on a couple errands. After she returned, I took the car out. The 'service engine soon' light DID NOT COME ON for a while. I only drove the car 3 or 4 miles, and just as I pulled up into my driveway, the light came on again. Also, while it was not lit, the car wasn't running rough any more. It seemed to run normally. Spooky, no? Any ideas?