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...
jsman, if you're using the KAB strobe light (and the batteries are good) you should be seeing a series of numbers that correspond with the speed the platter is spinning. So assuming you've got your turntable set for 33.33 RPM you should see a "33" displayed that is stationary (indicating steady speed) or floats slowly left or right depending on whether your turntable is running fast or slow. Not sure what you mean by " the lines on the strobe bounce back and forth."
Are you saying that the numbers should be stationary? That's what I have been doing wrong, I am looking at the bars. For whatever reason I thought the lines were to be stationary....
There are a lot of variables with measuring speed and W&F. That’s not to say you shouldn’t try to measure it, but take the values with a grain of salt. As an example, I get the following for the same belt driven TT:

RPM App/World TT Database: 33.33 RPM, +0.00%, 0.07 W&F
WFGUI: 0.0199% RMS W&F
Analog Magik: 0.0443% AES W&F
I prefer and feel more confident with the off board Strobe.
They usually cost the price of a Vinyl Album or Two, and don't get a lot of usage once utilised.
A Strobe built in to a TT can offer inaccuracies in showing the Speed,
it is worth reading up on some of the descriptions how this can be happening.

the RPM app for cell phones is accurate and a lot easier to use than a strobe. Check it out !