83 300 SD.. where does the tach get it's signal from ?

I have an intermittent problem where if I hit a series of bumps in the road or when it's raining, my tach either goes goofy or dead.

So I'm suspecting that there's either a bad plug connector or sending unit/switch involved somewhere on the engine/trans, as most of the time the gauge works fine except under those conditions.

I've heard something about a tachometer amp ? What and where is it ?

Thanks..