guide star problem

My System is Paramount ME, SBIG STX 16803 camera, AOX, ST 7 filter wheel and starchaser 3. I am using an RCOS 20" scope. I have Maxim DL 6.29. My problem is similar to others. When guiding east of the Meridian, Guiding is great. As I approach Meridian or flip to the west side of the Meridian, the guide star stars to guide, then veers off the trackbox screen suddenly in Right Ascension. The scope and mount are well balanced. What is going on?