Of late, my '88 240's speedometer has been pretty hoppin',
occasionally blipping up to 50 mph or so and bouncing a lot when
moving off from a stop (sometimes even when idling). Well, a few
days ago, the engine started missing on one cylinder under load in
wet weather. Tweaking the plug wires would temporarily cure the
misfire. I cleaned the plugs, which had some brown stuff on the
outside - carbon tracking? - checked the gaps, which were correct,
and reinstalled them. I also packed all of the high-voltage
connections with silicone dielectric grease. So far, knock on ...
errrr ... plastic dashboard ... this has cured both the skipping
and the speedometer bouncing. My theory is that high voltage was
arcing to ground and somehow causing interference with the speedometer
electronics.
.
-b.