I installed a large tach that I got off ebay in my 1992 245 a few weeks back. Initially the tach seemed to work fine. But now (most days) when I start the car out cold, the tach doesn't work at all, or jumps between 0-~2000 rpm. The engine is smooth the entire time, so its just an indication issue. If it smack the top of the dash, it sometimes comes alive for a bit, not always. After about 15 mins. it usually works fine.

Sounds like a bad contact or a failing instrument to me.

Have any of you experienced similar?

I do know that the gauge itself is getting power because I am using the power outputs on the back of it to energize a pair of aux VDO volt/ oil press gauges. These gauges always work properly, no fluctuations.

Thanks