After a few yards the trolley loses power and just about goes at crawling speed with no real power.
Lifting the driven wheels off the ground and lowering back down the speed and power returns for anything from 4 to 5 yards up to 100 yds.
Initial thoughts were the potentiometer but adjustment of the speed control does not change the fault when it occurrs.

Roy Ellis, November 2012

Had similar problem, found that one of the power cables under the battery tray was faulty. There was no obvious sign of this, but a current test of the wiring found there was a break within the cable.
May well be your trouble.

Peter Taylor, March 2014