Grautz drift is a common occurance in a planar simulation module, and can easily be rectified by adjusting the offset on your module using the oculation knobs. For the best performance, the readings on your planar correction module should display some number between -1 and 1. Any more or less, and you may need to adjust the knobs more. Professionals aim for exactly 0, however this is a difficult feat that requires more expensive equipment.