My 2003 D2 with 112k miles started having this issue with the tachometer needle bouncing all over the place although the RPMs aren’t changing. You can see the live readings on my head unit. I confirmed the voltage at the alternator with a multimeter and checked the two wires on the alternator. Would the next step be to check the wire behind the tachometer to make sure it has a good connection or should I be looking for something else?
Attachment 57701

 

Could be your CPS starting to go

 

I just put in a new one (Bosch) a few weeks ago.

 

Possible alternator issue - even a loose wire - check connections on back of the alternator and test the alternator could be going bad