I had a similar problem with Bachmann DMUs (running solo). It seemed as though there were phases of "free- wheeling" down the slope and then the motor slowing the unit down (as if in a lower gear). I solved the problem by inserting foam above the non-powered axles - as the 20s are powered on all axles I appreciate that this is not possible.
(Running DC using both Morley and Gaugemaster controllers.)