Ahmed A,
I know tthis sounds verty much like a mechanical problem, but I think it would be worth while checking the motor field windings for earth faults it could be that the motor is only running on two phases if that checks out ok the may you can run some inductance test on the phase outputs it may be that you have a power unit fault that appears to disapear as the truck system gets warmed up. also there is a bearing at the end of the motor that the power unit sences direction of rotation on but at the moment I cant think of its name but it may have become faulty. I think these areas need checking out.

Titus