[noavgdrvr]

My 1977 911s suffers from the same problem, although my tach works better when its cool outside. when i park the car outside in the sun on hot days the tach gets stuck around 2-4k. Sometimes it just rest at zero.

At one time it would correct it self with a tap on the glass but, that doesn't seem to work anymore.

I checked for correct alternator function and everything its fine. I think i need a new tach.