Fluctuating gauges

The fuel, water temp, and oil pressure gauges on my '74 260 fluctuate essentially as a group. What I mean by that is that they become powered and read, and then lose power. They get progressively worse with time till they don't read at all. The power to both gauges are common and fluctuates 11-13V but doesn't fail, and I've tried an alternate ground which didn't help. Inside each gauge is what appears to be a bi-metallic switch that has a heating coil of wire around it. These switches control the power to the gauge so I'm pretty sure they are the culprit for removing power. My belief is that the regulated voltage to the gauges is too high and these internal switches protect the gauges and are cutting out. I've done the regulator check in the manual, which apperars ok. The only thing I can think to go is replace the regulator.

Anyone run into something like this ?

Any suggestions ?