Have stock '72 z. When idling the temp moves nicely and stays at dead center. When travelling, temp gauge goes back to zero. What should I look for. I've already replaced the t-stat. Thanks.

Better that than going up to the red line.

I would have to guess it's a problem with the sending unit that screws in to the thermostat housing. I don't know why there would be a difference with the car moving, however.

My car was doing that and i could never figure it out.

One day while replaceing my condensor on my mallory distributor i must have bumped a few wiries and never had trouble again.

I never figured out what it was.

I guess you should look for bad connections.

Thanks guys...

I'll look into the sender unit and jiggle a few wires/ connedtions and hope for the best.

Yup...heading towards the red-line would definitely be worse.