Hi Kevin. and welcome to TGMF.
What I've found with these Weibangs is the double pully between the two belts has a very soft spring between the the two inner pully half's. This is supposed to be spring loaded to keep the belts tensioned. I've had a couple where the spring has gone limp and therefore the belts were running loose causing the symptoms you describe.
So I would check the two belts make sure they haven't worn excessively, and check that double pulley between the belts. There's a modified pulley for these now, if I can find the bulletin I'll post it up for you..