Nicholas Simons did an excellent job describing how to correct belt
slipping problems.  I would try everything he says first.  That being
said, <gulp> there is always belt dressing or an idler arm.  I am sure
that this will stir up a fire storm.  I will bet that the belt diameter
is too small.

Bob Yorburg

 [ The side walls of the pulley grip the belt to pull it.  The belt
 [ should not contact the pulley bottom as this means there's no grip.
 [ -- Robbie