Measuring TT Speed

In measuring my tt speed I am seeing the lines on the strobe bounce back and forth, not holding steady. This is a belt tt with a dc motor setup with a closed loop controller, and ideas on what can cause this? I am thinking the belt may be to tight, it is a new belt BTW...
Lewm, I used a strobe for decades and whatever error is involved is is meaningless sonically. Watching your telephone spin has a hypnotic effect particularly after a few gummies.

Well guys thanks for all the replies with some help from above and you guys I have it sorted out now, thanks....
The RPM app does have a calibration feature that is very easy to do , but you have to get the premium or paying version.  You also get the option of entering your tables in the database and compare it with others from around the globe. Is around 15 dlls ( i paid in pesos so i can,t remember for sure) but is well worth it.