What transformer are you using and how are you controlling voltage on Fixed 2? Check the motor leads and connections.
In conventional your getting full voltage correct? Does the engine smooth out in DCS at higher speeds?
Take shell off and watch the motors to see if one is dragging at low speed. Could be a bad motor, that is assuming that in conventional the voltage applied was high, were as DCS at speed 1-6 you only applying low voltage pulsed to the motors. G