Bluepen,

Once a belt squeals a few times it usually glazes and cooks which kills it gripping ability. If you have the proper number of belts powering each accessory, I would replace them, tension them correctly and give it another try.

Be sure to re-tighten them after a few days as they stretch quite a bit after initial installation. Look around and make sure oil or antifreeze isn't being blown on them from the fan or leaking on them somehow.

Cleaning the sheaves of all accumulated crud isn't a bad idea but I don't know is they need to be abraded. The rubber belt has tons of tractive effort, even on a smooth pulley.

Stuart