This is the problem I had, when the car was running the gauges and everyhting worked fine,but as soon as I turned on the headlights this happened,
Tach either jumped to 7000 or went to zero,
oil gauge went to zero,
volt meter went to zero,
right directional light came on, stayed lit up and worked with the dimmer switch along with the rest of the gauge lights that were working,
the lights behind the tach were either out or VERY VERY VERY dimmely lit.
As soon as turned the dimmer switch all the way down the gauges came back up and worked normally.
This problem, at least on mine was in the printed circuitboard on the back of the gauge cluster.
There must have either been a short in there or a break in the copper somewhere.
This I suppose could also be caused by a light socket that is badly corroded.
I picked up another gauge cluster from the local junkyard, as soon as I put it in and turned on the lights everything was fine.
I hope this helps people out.
If anyone needs any help let me know and I will try to help.