This may not be a thermostat problem. When was the last time the furnace was serviced?

Check the filter and see if needs to be changed. If you have a dirty or restrictive filter, the furnace could be experiencing a temperature limit override. This is a safety feature of the furnace so it does not overheat and damage the heat exchanger. Poor air flow can cause this problem.

I forgot to mention that our heat is 2 zones. The 1st zone on our 1st floor is working fine. The 2nd zone on our 2nd floor is not coming up to temp.

Anyway I was hoping it was something simple and we could fix it. We will call for service. Thanks mike for answering my post.