Ah, crappy parts right out of the box! Aftermarket junk..
The gage is driven by that sender in the cylinder head so perhaps it's junk, just seems like it grounds itself out and sticks. It doesn't control anything and nothing depends on it though. Tis important for obvious reasons, mine saved my bacon the other day. Despite having fresh coolant after 3 flushes, a new radiator, new thermostat and such my thermostat stuck closed for some reason and it overheated... :End rant:
You could check the resistance values of the sender at engine cold and engine hot and verify they are what they're supposed to be. I (naturally) forget what those are offhand. Cold they're some absurdly high resistance like in the thousands of ohms and hot they're really low, IIRC <50.
Have you tried a new computer yet? I'm not sure where your ESC lives, maybe on the later years it got integrated with the ECM? I went looking for spares and could only find them on '88-'90 trucks, as those years have them in the same spot mine is, in front of the TB to the left of it. Believe it has a wide 5 pin connector with flat blades.