From what you say I guess there is a poor contact somewhere, but if there is no obvious loose connection I'm not sure what to suggest. One source suggests resoldering all the connections as a solution, whether or not they appear loose. A fiddly job and from what I recall of the unit you would need to take care not to damage anything in the process. As for a replacement I know that 13BETC in the US offer a rebuild service or you can get a rebuilt unit from them....with properly soldered connections! Alternatively worth trying Tim at RX Motors.