Go to the basics:
Fuel: Check pressure at the rail -buy or borrow a fuel pressure test gauge, look for 33-42 psi at the rail, much outside of those numbers and you have a problem. You may also need a noid-light to check for injector signal. That is a little hard to do on an MPFI engine though.
Spark: Use an old spark-plug at the end of the ignition cable, ground it to the engine (don't touch it while cranking!) and look for spark, or far safer and more reliable for testing is to use a cheap inline spark tester. Verify spark from all three coils, use the front cylinder bank for ease of access.
Air: Make sure the engine can breathe, but this also means breathing OUT: If you had two bad injectors, they may have caused damage to your catalytic converter -fuel-loading can overheat the monolithic brick inside, and cause it to break apart, which may clog the exhaust system. Similar to having Eddy Murphy jam a potato up your car's tailpipe.
Also make sure the air the engine is getting is usable -a severe fault in the EGR system can prevent the engine from running, because it can dump dirty, unburnable exhaust gas into the intake.
Compression: Use a compression gauge, verify the engine is mechanically functional. I recently spent some time diagnosing a moderate misfire under load on a 3100 -the PCM could not identify the bad cylinder, but I found #5 was weak. After ruling everything else out, from defective spark-plugs (they were Autolite, so a very strong possibility) or coils to poor fuel pressure, I finally checked compression, and found the middle rear cylinder had only ~30psi.
While checking compression, make sure your overheating issue isn't caused by or the cause of a blown head-gasket or some other issue. One of the 3.1MPFI engines I've had over the years stopped starting because of a blown head-gasket. I literally drove the car around the block one time after I bought it, it started getting warm then stalled and wouldn't restart. That was also from a '90 Chev..... MPFI engines are less likely to blow head gaskets than a 3x00 in my experience, the heads are thicker castings which helps, but that might be what is going on with your car.
Obviously, a failed ECM could be the culprit too, not the most likely, but becoming more plausible with the age of our electronics, so don't rule it out.
It sounds like your car may have some neglect effecting it, so after some basic diagnostics, a good tune-up is a good way to 'clear the dust off' and may solve or reveal your problem. Don't forget to replace your fuel-filter while you are at it, and I usually replace the PCV valve at the same time, just to be sure it is done at some point -they can cause a lot of problems if allowed to clog up. Good luck, keep us posted.