Check:
- The tension in the chains is similar for each side
- The wiring to the motors on both sides has no defects (loose wiring, etc).
- Your wheels spin in similar fashions -- i.e. if 1 bearing is going bad due to wear/dirt/etc then it will cause that wheel to resist more, causing the robot to skew to that side.
It may be possible (but doubtful) that you have a worn out CIM motor (factory defective) and thus it is putting out less power. Check the motor Free Speeds if the above doesn't work.