VPI Rim Drive vibrations

I have VPI SSM with RIM drive and SDS. The issue I'm having is a vibration being transferred from rim to the desk/platter. As the result tonarm wobbles a little bit in azimuth plane.
I tried to replace the brownish rim belt, change contact force/distance between the drive and the platter, and reinstall the belt - the vibration still there (exactly one wobble per rim's full revolution - 360 degrees).

The speed stability is very good though - have checked it with KAB Speed Strobe.

Any ideas how to resolve the issue?

Thanks a lot!