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

You might want to check the connection at the coil.

I think you're right and maybe that is why the tach was on ebay in the first place.
When I put the Yellow Peril (73 145E) into operation I put a dash panel together
from several that I had and found a couple of tachs that would always start at 1000 rpm, even with the engine not running. I finally found one that seems to work right. I have not been inside the others but I do know that many relays and other components suffer from cracked solder joints and other intermittent conditions that are fairly easily fixed once you can see them. I'd bet that there is some sensitive connection in these tachs that comes loose, shorts, or otherwise messes up the operation. The fact that you can jar the tach with occasional good effect makes this a sensible conclusion.
Can you fix it? Dunno. Might be worth a try though.
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George Downs Bartlesville, Oklahoma