Strange problem with the heater in my 83
 

I was driving with the heater fan running on 4 and the temp on 85 when the warning light came on. the temp gauge went to zero and the heater went from heat to defroster even though it was still set on heat. I tried moving the heater controls and every once and a while the temp gauge would flicker up and go back down also I can hear clicking and dampers moving but it still stays on defrost. Could this be caused by the heater relays and why would that affect the temp gauge?
Thanks Otto

When was the last time you pulled all the relays and fuses out and cleaned all the contacts? Have you cleaned all the ground connections as posted in this site? I would do all that first and see what happens. Have you replaced all #53 relays with new ones?

I have cleaned most of the fuses and all the grounds I could find but not everyone. I did replace the relays for the fan and the defroster 21 and 22. Maybe I should just pull the fuse panel out and clean everything at one time. I've only had the car 5 months so I'm still learning but it sure is fun to drive.
Thanks Otto