I had that happen a few years ago. The VSS wasn't the cause, I swapped the cluster because I had another. About a year later it bounced again. I was going to swap this cluster with another, but when I was removing my ground strap from the battery it was very oxidized, so I cleaned it up. Fine since.

I haven't tried swapping the original cluster back in to see if it was actually a bad ground because my current one still works fine, so I can't be sure if it was from the ground or not. Just a long winded suggestion I guess.

Short version: clean ground points, check speed sensor, swap cluster.

It could have something to do with the dimmer switch also, seems like everything electrical on the dash is affected by that sucker. Then a couple turns of the dial and it settles down again (third switch).