Measuring TT Speed

Mijostyn, With much respect, especially as I have never used the particular application you recommend, RPM, cell phones are notoriously inaccurate for measuring TT speed.  If the strobe is wavering back and forth, it means something. Sometimes that can be an optical trick due to flickering of the strobe light or see below. So long as the strobe markings do not move definitively in one direction or the other, the problem may lie with the strobe.  The OP does not say what strobe he or she is using.  For me, the only strobe worth having is the KAB product, which comes with a battery-powered strobe light that is constant in its pulse and a larger than average diameter strobe, so the markings are moving at a faster speed due to a further distance from the center of rotation, compared to the average strobe disc.  Any strobe light that plugs into the wall is subject to the minute variations of the AC frequency and could give the phenomenon the OP is talking about. Unfortunately, the KAB kit with strobe disc and battery-powered strobe light costs $100, but is well worth it in my opinion.

Reality check: no belt, motor, AC, or anything else gonna make the lines "bounce back and forth" the changes in platter speed for that to happen are just insane, beyond the torque of the motor, and the OP wouldn't even need a strobe the table would be totally unlistenable. 

Its just light hitting the strobe bars at the wrong time. Ignore those, concentrate on the average, you'll be fine.

I am using the KAB light and disc for those interested...

Lewm, as far as I can tell the RPM app works great. I have a record weight with a flat top so I just put the phone on top of that. The phone reports in hundredths of a revolution. When I first tried it the phone reported a wow and flutter of 1.78 % which sucks. I got a new belt and wow and flutter dropped to 0.07%....on a 40 year old turntable. 
For speed it is certainly more accurate than a strobe. If I adjust it with the phone to an exact 33.33 RPM the strobe is dead still. It is cheap. I suggest you try it and tell us what you think! 

On the other hand, if you can't adjust your turntables speed it might piss you off.