1989 Chevy Corsica 1989 Chevy Corsica 2.8 L Help we're at w

We replace fuel pump with in a week of buying the car 2 years ago (were the 2nd owner) and then it ran fine for about 6 months. Maps went out we replaced it, but in the process the AIC went out we replaced that, then it ran fine 6 more months. Continued with monthly maintenances.
We parked it over the winter, charged the battery and found out the alternator went out, replaced alternator and it ran perfectly for 4 days and then the problems started! The tps tested good, we seem to fuel to the fuel rail, we seem to have spark. We replaced coil pack and ignition module. This all started with the car heating up to mid range then shutting off. We replaced the rear thermister, then it heated to half to 3/4s and would cycle fan on and cool engine down, this cycled 3 or 4 times then shut down again. Couldn't start again until cold. Now it won't start at all. When we did finally get it started it ran very rough. Then again. We we told by several mechanics, check the codes and it kept throwing a 121212 code. According Hane's and Chilton's manuals thats a diagnostic mode of all clear. Then we again were told now that the ECM was bad. Just purchased and installed the new ECM and still won't start, we can crank it over till the battery dies and still nothing. WE ARE STUMPED! We don't know where to go to next. Please if you have any suggestions we really need to hear from you! Sincerely Ginger Flores