I have a 2000 STS that started to heat up about 3 weeks ago, I was driving and it went to red and the DTC said to shut off a/c. I pulled over and shut off the engine. Restated it about 1 hour and it was fine right at 12 o'clock and stayed like that for a few days. Then it would jump rise to about 3 o'clock then go back down to 12 and be good for a day or two. Then it got real hot again and got the change oil and idle engine on the DTC. I am not losing any coolant that I can see and the belt for the water pump looks fine. Any ideas