One can also change the line voltage from the SDS to the motor and that often solves the speed instability issues. You can download the manual from the vpi website and there are instructions there on how to adjust the SDS.
VPI SDS owners-
I check the speed with the appropriate strobe every 4-6 weeks, or when I make a cartridge change. It was initially set at 60.12 when I got it (about a year ago). I would say I had to lower the speed 5 or 6 times since I got it. It now runs at 59.53 HZ to be spot on. Anyone know why this is happening? Should I be concerned? Turntable is the original VPI Aries1.