Unregistered

Hi All. I have a '91 F-150 with many miles on it. I have a problem in that the temperature gauge is quite erratic. It will stay on cold for quite a while after initially starting to drive, and then very quickly jump to nearly the hot side. I will have no heat from the heater either. Then, after a while it will come back down to the "normal" range closer to the cold side, and the heater will blow warm air. But, after another few minutes of driving (or shutting the truck off), it will do the same trick again....high temperature gauge - no heat.
Any ideas?? This is frustrating.
Thank in advance.
Ken
BTW - the thermostat was replaced about 6 months ago. All was good until this started happening about 2 months ago....