Tach works intermittently. Other times jumps around.

[FuddleyPJones]

Hello folks,

This is the first time I've owned a Volvo. My recently deceased brother left this behind.
It's a 1997 960 V90 wagon.
(I honestly don't know the difference between 960 and V90 but there seems to be something).

Anyway there's several things that I'm having issues with but one of them is that the tachometer does this weird thing where it will work just fine and then either over time or at the turn of the key and restarting the tach is dead but when you rev the engine it jumps from 0 to 4,000 and then you let off the gas and immediately drops down to zero.
I turn it off and restart and the tack works again - sometimes...
I saw a suggestion where I should try wiggling the key as that might be an indication that the key switch is on its way out - But wiggling seems to have no effect on stopping the weird tachometer behavior and making it work normally.

Any ideas?

[lummert]

Any ignition system symptoms? Like misfires.

[FuddleyPJones]

No issues. No missing. No stalling.
Drives fine. Tach will jump from 0 to 4000 and back randomly. Other times it works fine.
50/50.