You nailed it -- a malfunctioning ignition control module can be hard to diagnose because they can test perfectly fine when cold. I was having engine trouble a few years ago and it took the mechanic some time to figure out what was going wrong. I thought the car must have hated me because it worked fine for him but would fail on me.
It turns out the ignition control module had gone bad. It wasn't too expensive to get it fixed, but getting to the point where the mechanic figured out the thing was broken was aggravating.