Jubie,
MB tops (and MB cars) are very complicated. Try to find a good MB mechanic you can trust. Research, network, pick local brains, whatever, but try to find someone in your area that knows these cars. Try here:
http://www.peachparts.com/shopforum/forumdisplay.php?f=14
The dealer can be a good place to start for diagnosing but be very wary of throwing money at it if the prognosis is flaky. IE: It may be necessary to pay for troubleshooting but don't start throwing parts at the problem unless you have a good reason to believe a fix is at hand, ("Well, let's try this, and if it doesn't work, we'll try something else! "). A good diagnoses is worth a lot. You said you took this to 3 different places. Unless you mean 3 different shops that specialize in MBs, you are wasting your time. Also, it may help to post in your public profile where in the world you are. Help may be nearby, if only we knew where nearby is.
Aaaah, 91 500sl, a very nice car. If I had one, and it was early summer, and the roads were beckoning, and the top was misbehaving, well, YOU know...
Good Luck, Dan
Please don't forget to close with us when you finally resolve this. Some other poor soul will undoubtably have the same problem someday.