Erratic tachometer (rpm gauge)
 

My '93 has an erratic tachometer which jumps 1-2,000 rpms back and forth, but with no change in throttle position, or audible from the engine, or any sense of acceleration. I'm pretty sure its a ghost, and not actually jacking up the RPMs by 2000 in .03 seconds.
Let me give a specific example: cruising at 45mph, 4th gear, 3k rpms... tachometer will suddenly jump to 4k and then drop right back to 3k, like a twitch.

But I yield to those who may know better than I: is this a bug that others have seen? Where should I start looking; gauge cluster or elsewhere?
I can replace the gauge cluster but before I do I'd like to have some sense that the problem is in there (and not in the engine or trani or something)... and I have no intention of letting a mechanic spend 12 hours trying to diagnose it.

What say the experts on here?

probably a ghost like you said. theres a few psychics in my area I had to call in to get rid of a ghost I had in my car. the ghost kept popping my gas door. I got ahold of the psychic and he talked to the ghost. the ghodt told him he just wanted to make sure I had gas so i wouldnt get stranded. we decided to let the ghost be so I will never have to worry about running out of gas. if I were to guess id say your ghost is trying to communicate with you and needs your help getting out... OR you have a bad connection somewhere, im betting on the 1st thing.. maybe the 2nd. look in the FSM/body electrical manual.

If it is stock and running fine with no other issues afraid you probably have a tach going out. I have replaced a few.
Check to see if you have any codes.
Check wire loom for coils, but usually if there is a problem there you will have other problems.

I KNOW I KNOW (jumping up and down waving hand)
the solder joints to one of the capacitors is getting bad,
here ya go
http://www.wrex-racing.com/docs/fd/Tach_Repair.pdf
if you don't feel like doing it yourself, remove the tach as per the instructions and take it to a local computer store that you can trust to be careful and have them do it. The capacitor is very obscure and the guy that's doing mine for me right now is still waiting on them (there are two he's replacing) to come in.

I just had this same problem and found a ground wire loose in the engine bay near my driverside headlight. Tightened it up and haven't had any problems.