[Arran]

I think that there may be some conductive crap in one of the trimmers, a mixture of dirt, rust, and mouse pee pee. Believe it or not I usually wash chassis when they get very dirty, TSP is very good and getting rid of dirt and oxidation in things. The problems you are having with this 42-761EZ are precisely the sort of problems I would expect to find in that rusty 38-690 that was on fleabay, except that one had evidence of some very pitted aluminum parts as well as rust.
Regards
Arran