My '92 used to do it when I was going highway speed and hit the turn signals, the speedometer would dip way down then snap back to where it was. It's been -forever- since I researched this problem but my very vague memory of it was there was a TSB about adding a small capacitor to the gauge cluster circuit board to act as a buffer; good luck finding that info but I also seem to recall it resulted in a design change later. I wouldn't suggest based on my super vague memory that you run out and buy another cluster but IF you have easy access to say one from a '94 it might be worth trying just on a whim.
Also as TechNova said check all your grounds particularly the ones at, and next to, the thermostat housing...make sure they're top notch. Also your gauge cluster grounds on the metal tab that your ALDL (diag) connector is attached to, so check/clean that one up, too. Single black wire with a ring terminal.
If all else fails it may be your speed sensor in the transmission tailshaft housing.
Richard