Quoted from KenH:

I worked on a JP that had this problem. The l/r movement was so sluggish, that the trex could not find his center before the CPU would disable the l/r movement.
To correct the problem, I added full-wave rectification for the l/r motor, which greatly improved the trex l/r movement. If you look at the circuit board above, you'll see 3 diodes. 2 are for full wave rectification of the shaker motor, and 1 is for half wave rectification of the trex l/r motor. Since the AC feed for this comes from a center-tapped part of the transformer (see manual pg. 46), you can easily add one diode and one fuse to the circuit board above and feed a full wave rectified signal to the l/r motor (see page 49, left side). If you want to change it back to half wave rectified (original) later, you simply remove one of two the fuses for the l/r motor.