Are you sure all the pulleys are turning? Is your belt adjusted to the right tension? Check the belt tensioner. Like Larry said DO NOT put anything on the belt. As a matter of fact, check if the belt has anything on i.e.oil, antifreeze, etc.. I just re-read your post. Did your problems started when you first starting washing the engine for salty roads? If your answer is yes, that is your problem. ANYTHING on your belt is bad.
Let us know...

__________________
J.H.
'86 300E