Cooling problem is definitely from your thermostat. Your ECU does soundn like it's causing most of your driveability problems. What you should try to do is find an ECU out of a '94-'95 GSR, that will run the car perfectly. The fact that you don't have any codes leads me to believe that either you have no wiring issues, or you have a really badly chipped ECU that disregards error codes and doesn't turn on the CEL (some have been known to do that).