Erratic temperature gauge readings

The temperature gauge on our 2000 Silhouette (venture sister) goes up and down erratically and the heater goes on and off. The radiator is full of coolant and the overflow bottle is at the correct level. Does anybody have any ideas on what could be causing these problems?

When you say heater goes on an off, do you mean it blows hot air then cold? If it blows hot air when you are moving but then blows cold air when you stop, I'm afraid that's a sign of a blown head gasket.

paprius4030, thanks for your reply.

The heater changes from blowing hot air to cold air, back to hot etc. I have not noticed it to be linked to driving or stopping.
I was leaning toward head gasket as well and was trying to get some feedback from other forum members.
For those that have had the head gaskets changed by a mechanic, what is the approximate cost. Hopefully there is no damage to the heads as the temp gage only really made it up to 3/4, with the exception of once to the red for about 10 seconds and then back to 1/4 (dash light did come on briefly)

Thanks,
Mark

GM labor guide pays about 7.5 hours for the job (both heads) ....multiply that by the shops' hourly rate + parts & tax = da bill. I'd guess around $700....

Thanks Kornjulio for the estimate.
I might get off easy on this one. I had it at the mechanics today and he thought it might just be a bad thermosat. I'll keep my fingers crossed!!
Mark

Keep us advised how you make out.

Well, it's been two days now since thermostat has been replaced. Everything seems to be working well so far. I will still keep my fingers crossed for a while. It still puzzles me how a stuck/bad thermostat would allow air in the system.
Any ideas?

Thanks,
Mark

Glad you seem to have lucked out