2002 Buick Rendezvous -Hot Coolant Temp

I have a 02 Buick Rendezvous CX with 95,000 miles V6. The Hot coolant light tends to have a mind of it's own . The needle will start to head to the Hot (red) and then go back down toward cold with no rhyme or reason - sometimes staying on hot and I have to shut off the car to cool it down. While the needle is on the hot side, I have no heat! It's weird. Then when the 'Hot Coolant Light' is off and the needle goes back toward cold, I have heat again. But, blasting the heat (because it was getting cold) tended to make the temp go up! ??? This temp thing has been happening more often, but sometimes there's no problem at all! The Dexcoolant fluid is filled and not leaking. What do you think?

Thank you,
Kathy

 

I suspect you have an air bubble in the system that would explain both intermittent heater output and screwy temp gage.