I've had my E6K in my 3rd gen for about 15 years. Sometime ago the tachometer started to just bounce around It's erratic at idle and will go past redline under moderate throttle.
I've confirmed it's not the tach itself (swapped clusters)
The car runs perfect nice steady idle and pulls strong.
Any ideas?
Thanks.

 

does the tach jump on laptop - haltech software (= problem with CAS pickup) or only on dash? (then its probably your haltech tach output wire, unless you have your tach wired to one of the coils directly....

since you say car runs ok, i would suspect second option