What is the rating of your power supply, and what do the motors draw?
It's possible that the motor load at full power is exceeding the supply rating.
Another thing I would check is brush noise (assuming these are the typical less expensive DC motors). Brush noise can cause all sorts of headaches.
A NPO cap across the motor terminals helps squelch the noise (and also extends the life of the brushes). Keep the leads as short as possible.
Choke inductors in series with the motor leads also help a lot.
Its possible there is some 'unintended behavior' (bug) in the PS2 library, but given that it 'almost' works, I would check these things before delving into the library code.