The motors in this video are the older kind with gears rather than drive belts, but adjusting them is exactly the same, so ignore the gears, if you don't have them, and pay attention to the screws being adjusted by Jim Higginbotham.
Adjust the height of the motors when iQ is either turned off or set to manual mode. You need to get down and look at the gap between the drive wheel (that's the flat sided wheel under the motor). Ideally it shouldn't be any greater than the thickness of two credit cards. The wheel should be completely over the surface it runs along and the gap should be even. If your gap is wider at one side than the other, alter the appropriate screw to level it (the silver ones in this video)
This can be a time consuming job, but you will be able to do it. Don't be tempted to loosen the screws more than a quarter turn at a time - do the adjustment just as Jim shows, checking the height after each adjustment.
When you have the motors adjusted, you must calibrate iQ, and run the drift test just to make sure everything is correct.
