I have a VPI HW-19 Mk4 turntable that sometimes experiences the same symptoms you describe. Assuming there is nothing wrong with the motor, I suspect that the drive belt is slipping. The VPI drive belt needs to be periodically dusted with talcum powder so there is some friction between the belt and the perimeter of the platter. Simply put a little baby talc in a ziplock bag, toss in the drive belt, shake it, then remove the belt and knock off the excess talc, and reinstall the belt.
