Since it was not mentioned in this thread yet, here’s the ultimate how-to for proper belt tensioning:
120-140Hz is my new motto.
And then, inspect your v-wheels very closely for integrity (do they still look round and smooth), and then I would say loosen all v-wheels and re-fasten the eccentrics such that the wheels don’t slip anymore…but just barely. Too-tight vwheels can do strange things, if some of the other threads are any indication.