This problem has me totally stumped so I am hoping club members might have a suggestion.
HISTORY. Driving car one morning and car stopped running. Think of throwing a light switch to OFF. Car would turn over but no go. Had car towed to work for mechanics to look at. Here'* what we found...
1)Metal shavings from water pump replacement covering crank sensor-part changed
2)Wiring harness from ignition module, cam and crank sensor leading to plug behind alternator in crap condition-found used harness in good condition
3)Replaced ignition module
Vehicle still no start. Was told PCM was bad. Replaced with remaned-vehicle runs but very rough
Currently-Code 14 (Coolant sensor/high temp) and 44 (Lean exhaust) now showing up in computer when they were not there before. Car runs like a bag of crap now. Cleared codes but SES light does NOT go out. When I put car in OD, D, 2, 1 and step on gas the RPM'* rise as per normal but car creeps ahead super slow and I dont think its shifting right if at all. When car in this mode I have no speedometer. After a while as if that switch is thrown the car goes normal for a few minutes then back to its crap self. Any ideas what I am missing here to look at?

Check your O2 sensor and make certain that it has not grounded out or the wires came in contact with the exhaust. When was the last time your replaced the sensor?

Disconnect the MAF sensor. If the car runs better, then clean the MAF and try again. If it still runs rough after plugging them MAF back in check this thread and test the MAF https://www.gmforum.com/t279320/

Check for vacuum leaks and repair them. Spray carb cleaner around vacuum lines while the engine is running. If the engine revs higher when you spray in an area, that where you have a a leak.

The O2 sensor is about 6 months old. After using Bosch and replacing it 3 times I went with ACDelco. The wire is far enough away from exhaust that it shouldn't be an issue. I have looked for vacuum leaks and replaced some lines. Will try the MAF sensor and see what happens. Have done some reading and I should mention that I did replace PCM but used old PROM chip. Am leaning toward replacing it as well to see how car runs.

You mention swapping the computer. If you didn't already, swap in the blue chip under the little cover held on by the two screws. That'll have the programming for your engine/year versus a possible different pcm programming.

Check the fuel pressure. Possibilities include: failing fuel pump; clogged fuel lines or filter; intermittent open in the wiring in the fuel sender assembly (which includes the wiring for the fuel pump.)