[holygrail]

We have almost the exact same setup and we were slipping belts too. We were able to fix it by doing a few things. We haven't slipped a tooth since.

1) We added a 3rd pulley. We needed more teeth in contact with those small 18 tooth pulleys
2) Added shaft support in the center of the driven shaft. It helps minimize any potential bending of the shaft which can change the tension.
3) Improved belt tensioning. We used a Turnbull from the hardware store and it worked great.