Check your drive wheels for damage or flat spots. Clean thoroughly. Also clean your wheels and rails. You may need to run a motor calibration. An instructional video for calibrating can be found here: http://www.iqdemos.com/search/label/Calibration
If you suspect that one of your motors is off, you can enable the Motor Movement Monitoring (Main Menu>Utilities>Preferences, it is the last option). When in the stitching screen, hit "Sew Quilt",and you will see the "Show/Hide Trace" button show up in the middle ribbon. Hit the "Hide/Show Trace" button and watch the trace line to see if it follows the pattern. If it does not, you may need to adjust your motors.