Tach needle jumps - 84 380SE
 

All:

When on the freeway and travelling at speeds which should register at around 2500 ~ 3000 rpm on the tach, the needle every once in a while will jump to 4000 rpm and then come back to normal. It is as if the needle was given a kick by the sensor.

And if the jump is higher than 5000 rpm, the engine wants to cut off.

Any idea what is the problem ? Do I need to tighten the tach sensor or something ? IF so, where is it located ?

Thanks,

Amit