86 IROC, Gauges stop working

When the car is cold all dash lights work. After driving 10 to 20 minutes the tach,oil pressure gauge and lights for these gauges stop working. When the heater is on they go stop working sooner. If A/C is on they stay on longer. I have changed clusters and same thing happens. One person told me he had the same problem until he changed the heater coil. After that they worked fine. I think maybe its a short some where. Has anyone had this problem if so where was their short. Any help would be appreciated. :banghead:

Sounds like just what you described... a heat related short