VPI SDS owners-

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.

Sounds like an isolation issue, that is over time your table is moving around a tad. Slight movements when you change the cart... just a thought. Btw I have the same table and my SDS is always spot on. I also use a 1000g clamp. Both the table and the SDS have their own stand. I can wrap the table hard with my hand while playing an album and you don't hear it...

Go figure.

I had the same challenge trying to keep things in sync with my SDS. Although not a complete fix all, things are much, much more stable since I've had the SDS plugged into a PS Audio PPP.

Good points, Tom.

I believe those issues applies to every belt drive turntable with the exception of the direct drive....perhaps, which I have no experience with.

Ah, the SDS... I can't live without it, yet it drives me bananas. I've reached the point where I set the correct speed, using a strobe, before each listening session. I've found that the speed is rarely correct 2 times in a row.
I've not been too scientific in my study of this, but here are factors that seem to be influential...

1) Weather. The big one. Unless your gear is set up in a room that has exquisite temperature control (mine isn't), the weather (temperature AND humidity) will affect your speed. Colder = Slower. Did you initially set up during the winter??

2) Mains power. If you're getting stable 120V, this should not be much of a factor, but it still is a factor.

3) Quality of belts. Once the belts start exhibiting those small cracks, speed stability is very unlikely.

4) Motor distance to platter. If it changed at all, speed would change.

5) Warm-up. I find that, after an hour of playing records, I'm usually running slightly faster than 33 1/3 and I re-adjust.

I hope that helps. If all else fails, check the batteries in your strobe :>)

Tom