Thanks for that offer! It is indeed a 26" wheel (the MTB size rim, with a Compass RTP tire), so your offer would work; but though I'm in NYC every day, the bike doesn't get closer than the Hamilton, NJ, NJTransit station.

Yes, I've tried jiggling the wires at the light, as well as just about everywhere I can think of. Checking for shorts is really difficult if the problem is intermittent.

Trying the bike with a different wheel is a good idea. I have a 650b wheel that I can pop in there; the brake won't work, but if I can diagnose the problem, then I'll be on my way to fixing it.