Latemodel57

Hey guys, I just bought a 91 GT for a fixer upper. The problem is that only the speedometer works untill the car gets good and warmed up and then all the sudden the seat belt light comes on and the rest of the guages come to life. This will only happen if I am going relatively slow, like in residential area. As soon as I get on the Highway, the guages including the tach quit working again. Anyone else had a problem like this? Thanks a lot