AX3500 issues

We have AX3500 channel controller. We set the controller with motors of 300 Watts and relative encoders of 300 pulses per revolution. The problem is that the controller was crazily running with full power in PID control mode. It was working anyway under the parameter of P=5,I=0, D=1., but it had low torque as it had stopped under weak friction. When the parameter changed over 5 of P gain or small number of I gain(0.1), the robot rushed forward. We tried many things, but we just figure out the phenomenon. Were we miss something? or Does the controller have a bug?

Try first to operate the controller with Roborun in open loop. Verify that you can move both motors at any speed in both forward and reverse.
While in open loop, click on the Encoder tab and verify that you see the encoder count and give speed information. Verify that it counts up when rotating forward and count down while rotating in reverse.
Make sure that you have a good time base value so that you get a reported speed value of ~120 (or so) while at full speed.
If all the above is OK, then you can play with different PID values. Note that if you are trying to control speed, then the I parameter is the most important.