VPI SDS speed issue

Input line voltage/frequency fluctuations to the SDS should not change the output frequency - that's the whole point of the unit, it synthesizes a stable output frequency independent of the line.

When I was experimenting with using dental floss as a belt material, belt slippage caused all sorts of problems. As slippage got worse I had to keep increasing the frequency to get proper speed.
Try some replacement belts and it may cure your issues.

Regarding SDS output voltage. While the factory setting after startup is 72V, I actually run my SDS at 115V all the time (set per the procedure in the manual). I find this sounds punchier in my setup - but I use the original heavy TNT lead/acrylic platter with the single motor/flywheel, which is not a stock setup.

As pointed out by myself and Brf, the output of the SDS is independent of fluctuations in frequency of the mains line. The SDS output wave is produced by an adjustable oscillator driving an amplifier. I would not expect much oscillator frequency drift from such a setup - and if it does vary slightly, it won't be related to mains frequency drift.

I understand about having to make small frequency adjustments on the SDS from time to time. I suspect this may be more due to mechanical fluctuations with the motor/bearings/belts etc. This is no big deal. Since we're talking about small changes over long periods of time, it should have little sonic consequence.
If you are having more frequent issues, the top suspect would be worn drive belts IMO.