Class 25 PC board problem

[Austrag]

My Bachmann 32-401DS  Class 25/3 has a lighting problem, the head lights work but after a about 3-5 seconds one direction fades. Switching the them over for around the same period of time  they come on and fade again. I have cleaned all the contacts several times until they gleam and also I have re-soldered every point on the wiring even the components have been redone. I have also switched the sound decoders as I have two locos the same but the fault is only on one of them. I am of the opinion that there is a faulty component, possibly a capacitor. There are only a half dozen but which one is a mystery. Has anyone had a similar fault and how was it solved please?