Instruments Intermittent

I'm not familiar with the 64 cluster. Is the IVR in the fuel gauge in those? There are several "gotchas" in the 67/ later stuff.........

intermittent power to the IVR
Those fit into a "socket" in the pc board, those contacts can give trouble
studs/ nuts on the gauges can lose contact and get corroded.
Of course the IVR itself.