Tachometer anomalies

I have an 83 300D Turbo. The Tachometer is dead except once in a while it moves at an erratic rate usually between shifts or at higher speeds on the highway. I tried the cigarette butt under the amplifier cap, then resoldered all the connections on the circuit board (took a lot of time to remove all the silicon potting). It still does the same thing. I can see the pick-up for the amp going to the front of the engine but haven't been able to see where it goes. Does anyone think the pick-up might have come loose? Any recomendations on how to check the pick-up for looseness? Thanks in advance.