Erratic tachometer (rpm gauge)

[adrock]

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?