Bad contact in the 3-wire connector tothe rear of speedometer??
Maybe spray-clean with "TV tuner cleaner" and maybe pinch the contacts a bit with an appropriate tool so they grab just a bit tighter. But be gentle.

Also have a look at the ribbon-like "flex" thing that goes across the rear of the speedometer. I just had mine out today ('89). I've always believed this is a fuse. Anyway, the copper conductor is on the bottom and does NOT contact the two screws that hold the ribbon in place. Testing for continuity at the screw heads, I thought the ribbon was bad. Removed screws, found copper on underside of ribbon, tested it, it was good. You can see that the copper contacts the circuit board where the screws hold it down.
--
Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).