Check all your Axes, make sure every bolt is tightened and the frame is solid. I was having similar problems with my prints on a CTC 3D Printer, and I made sure to oil/lube up all the axes and tighten everything up properly. I found out the primary cause of my issue was the angle brackets holding my glass bed in place were slightly melting as the heated bed temp went up, causing the bed to shift back and forth as the Z level moved.