Eldartford,
I believe (based on prior threads) that Tfkaudio may have a VPI Scoutmaster, which uses a synchronous motor with a relatively small (1 to 2 inch diameter) plastic pulley on the end of its shaft, driving a rubber belt that goes around the outer rim of a full 12 inch plastic platter. I trust someone will correct me if I'm wrong.
The SDS is basically a monobloc amplifier that takes wall power and "plays" a single tone. That tone is a sine wave with frequency of approximately 60 herz. This frequency is adjustable, i.e., you can set it somewhat plus or minus from 60 herz. We can observe the effect on platter rotation by using a strobe disc on the platter and accordingly adjust the SDS frequency so the platter rotates at the correct speed.
The reasons why the frequency might need to be adjusted include system friction and reduced motor effectiveness. For example, if I press my finger against the outer rim of the rotating platter, it will slow down. This may be due to slippage somewhere in the system, most likely from the belt, or it may be due to insufficient power from the motor (just like pinching the rotating motor shaft hard enough can slow the rotation). As you can see, this configuration has some vulnerabilities compared to your direct drive.
If, for example, there were too much friction and the belt were slipping slightly, increasing the SDS frequency would offset this by setting a faster pace from which the slippage would reduce the rotation back to the desired speed. Admittedly, not the most elegant solution, but it works.
Slippage may be variable, depending on belt material characteristics relative to environmental factors. The Scoutmaster configuration requires the belt to flex and stretch a little as it goes around the pulley and platter. Perhaps the belt gets stiffer when it is cold, and this could result in greater slippage. Or, the slippage may be a function of the humidity of the air, which could effectively lubricate the belt where it meets the pulley or the platter. Another possibility is that the platter's bearing lubricant has temperature dependant viscosity. (I might guess that his lithium grease is not the lubricant recommended by the manufacturer.) Maybe Tfkaudio has a bit of foreign matter in his bearing that requires low viscosity lubrication to overcome. Obviously, I really don't know.
If his turntable's performance does not change significantly from where it is now, I would think knowing the cause is not so important. If it keeps changing, I would recommend contacting VPI and having them deal with it. Perhaps they may have encountered this before.
