Having some weird issues with my tracking (Advanced VX) I need your help! I am having some weird tracking issues with my tripod (Advanced VX). When I setup and calibrated my scope yesterday I took this picture of NGC6888 (90s exposure): 
As you can see this image looks quite good tracking wise. 

However due to unknown reasons it seems that 1 in 3 images is always bad and has tracking issues, like this one: 
This movement always occours in the same direction, it seems that for some reason one axis of the mount is moving faster periodically, causing these tracking issues.

Funny is that is seems to be occouring periodically. I always get 4 images good, 2 images bad, 4 images good, 2 images bad, ...

I tried fiddeling around with the anti-backlash settings (I never touched them before) but that didnt fix it. My current anti-backlash settings are: RA +- 55, Dec +- 60

Does anyone know where those tracking issues could come from and how to fix them? Note I also have a 5s mirror-lock so it should not come from vibrations of the camera.
Looks like you have a periodic error issue. The worm period on the AVX is about 9mins I think. The issue has a period of 90sec * 6 = 540sec (9min).