2002 Other Oldsmobile Models heater blowing cold and overhea

I have a 02 Olsmobile Silohette and recently the heater started blowing only luke warm to cold air and my temp gauge is going all over the place and overheats if the car isn't moving. The heater blows luke warm when the temp gauge shows ablut 1/4 and then blows cold when it gets up to 1/2. We replaced the thermastat and it is still having the problem. Can you help please.