I've seen this lots of times on the wp3200 it is a common fault, it will either be a faulty traction motor encoder (I've replaced lots of them) or a broken wire between the endoded and access 3 (another common fault) is the drive motor getting hot??. Basically access3 doesn't know how fast the motor is turning so just gives it max current so it nearly stalls the motor, either issues I've mentioned will give the symptoms you describe