1997 Chevy Monte Carlo

I would check the water pump and lower intake gaskets for possible coolant leaks. The temp gauge is most likely irratic because air is getting into the system because of the leak.
Although if you have severly overheated the engine at some point due to the lack of coolant, you may have warpped the cylinder heads.

Check the coolant level when the car is cold, after sitting all night. Remove the ratiator cap fill that up, then also fill the over flow bottle. See if that settles down the temp gauge and gives you some heat for a while, and let me know how you make out with that.