The belt contact area of the pulley could have gotten polished from years of slipping to the point the belt can't get enough traction. Try removing the pulley and have an automotive machine shop beadblast the belt contact area to give the belt something to grab.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!