Faulty thermostat?

Hello again,

I have a 2000 528i. Once in a while I will glance down and see the
temperature guage in the red. My first reaction was the car was
overheating, but then suddenly the needle went back down in the normal
area. Most of the time when this happens I had just started the car. It
happened yesterday while I was stuck in traffic. I know the engine was not
overheating becuase the needle will go down very fast.

Is this a thermostat going bad?

Please advise,
Jeff