I have a 1995 850 n/a, and the tach doesn't like to work
some days (particularly warmer days), my gauge will work fine, others, not at all, and sometimes, the needle will bounce from actual RPM to 0 and somewhere in-between before dropping back down to 0

everything works fine, and the car runs great without the tach, but I would really like to be able to see my RPMs and be able to shift more smoothly. It seems to be a loose connection somewhere, but I have no idea where to start

The cluster is a Yazaki, and I haven't been able to get it to go into the test mode through the OBD I system

 

That is a good sign of your ignition switch going... are yo u experiencing any other issues such as your lights not coming on or delayed seatbelt warning ding?
What happens when you jiggle the key when its not working?

 

none of those problems
it decided to work this afternoon, so next time I go out, I'll try that.

 

messing with the key did nothing

 

I also can't pull any diagnostic codes for the cluster through port A7 on the OBD system
tried the 3-press test mode
nothing
tried the single-press code puller
nothing