If you have a stitch regulator, check and make sure that you are in manual mode instead of regulated mode. Check all wheels including drive wheels for dirt or damage. Also check O-rings for cracks or wear. Check your motor brushes for wear. Drive wheels, motor brushes and O-rings can be purchased from www.quilttech.com.
Check your motor speed and make sure that it is set high enough.
Look for loose wires that may be giving an inconsistent signal.