I have one of these, I bought it as ‘new old stock’. As it sat unused for years before I acquired it, the drive belt inside the gearbox was a little stiff and because of the memory of the rubber, it would slip on the pulley wheels. I’m guessing your drive belt might have some memory baked into it. I removed the belt (picture a flat rubber band), flipped it so the outer side of the band became the inner side of the band. Works great now.
You have to remove the house to get to the gearbox. The gearbox was a little tricky to open, but if I remember correctly, it just had some tabs to release to open it up.
