Hot end suddenly not heating up

I have a home-built MendelMax 1.5+ that's been running great for around a year. RAMPS 1.4, 24V PSU with a 12V stepdown to run the hot end and fans.

After a long print on Saturday, I turned the printer on on Sunday but found that the hot end would not heat up. Here's what I've observed:

1) The hot end thermistor is fine. I measure a good resistance across it, the software is reading room temp and touching the hot end warms it up enough to register the difference.
2) I get 12V across the terminals of the stepdown output before turning on the heater. I have a fan over the extruder stepper that is hooked directly up to the 12V terminals, so simply turning on the power turns this fan on. It's running fine.
3) When I turn on the heater via the software, the fan I just mentioned stops running, I hear the PSU fan adjust slightly, but the hot end simply doesn't heat up.
4) Last night while measuring with my multimeter, I don't know if it was coincidence or something I touched, but the hot end suddenly kicked into life. I turned the printer off and on again, but it was no longer working. I haven't been able to reproduce.

Just to be clear about my setup, it's like this figure: [farm9.staticflickr.com]
The only addition is that I have another fan connected directly to the 12V terminals, as described in #2 above.

I'm out of ideas. Can anyone suggest some more troubleshooting steps?

Thanks!