Troublesome Speedometer

[deviousKA]

ok. i have a 1990 240. my speedometer will work sometimes and sometimes not. it will (about 40% of the time) stop working and drop to zero. i took my dash apart and pulled the guages out expecting it to be similar to a d21 analog speedo, with a cable. to my surprise i find that it is an electronic needle unit. i have a crappy haynes or chilton book and i shows that unless i have a digital speedometer i should have an analog cable. ok i thought i was going to be able to repair the problem by fixing a falty cable connection as tapping on my dash seems to sometimes fix the problem. is this a common problem? anyone have a fix? im probably just going to the wrecking yard and getting a new guage cluster. maybe if i unplug it and reinstall it will work? i kinda doubt it though