Intermittent tachometer

[gearheadE30]

Anyone ever heard of an intermittent tachometer? Mine drops out once the car warms up. It seems to be based more on engine bay temperature than it is on coolant temp itself. It takes much longer to fail if I'm running down the highway than if I'm just driving around town. Figure about 40 minutes vs. 20, respectively.

When it goes, it's usually not sudden. It will start jumping around, going between 0 rpm and whatever the actual engine rpm is. There doesn't seem to be a pattern to the bouncing needle, and for the most part there seems to be no correlation with bumpy roads. The only control variable I could find is throttle position - when it is in that jumpy phase between working fine and not working at all, if I lift off the gas, the tach will come back. If I stand on the throttle, it will shut off. This seems to be very repeatable.

This seems to me to be a failing crank sensor or something, but there is no change in how the car drives. I know there are two crank sensors on the EG33 - is one of them dedicated to the tach? Any other things that could cause this?