Ah, thanks for the clarity.
I had this happen with my original lathe, a Nova Comet DR14. Same issue...RPM was not steady just with the motor on and no load on the lathe. Teknatool insisted it was the control board, which they sent me at least 5 of. After replacing the control board four times, I told them that definitely wasn't the issue. Took a couple months, before they finally agreed to take the lathe back on RMA. I didn't hear a thing from them until several months later (had written the darn thing off, and bought the Wen lathe I now use for my pens in the intervening time!!) They finally sent the lathe back and emailed me near the end of January 2021 (the problems started summer of 2020), and said they rebuilt the entire thing, replacing one part at a time in the headstock until they finally replaced the motor, which was the problem.
They sent it back with this incredibly loud scraping sound in the headstock. I've not used it since.
I had a wavering motor problem with my Laguna Revo 1524 as well. Again, I was told it was the control box. Replaced that. Ultimately they ended up agreeing it was the motor. Replaced that and it was fine.
I've done extensive power tests in my home and workshop. I have a ROCK SOLID 125V, with minimal droop when powering devices and machines. I've done long term power usage measurements, and the power has always been incredibly stable and reliable. So I don't know why I had motor issues with two lathes. Since replacing those two, I have a Wen and a Powermatic, I've had no issues since. But, I have had exactly the same problem you described, wavering RPMs (and you can hear it in the motor pitch as well) without any load, and also even more significant drops when the motors were loaded. In the end replacement motors was what fixed the problems.