Could it be that you share a pole transformer with a neighbor or two? Ask Noam about the CFL across the street that messed up his X10 system.
No need to ask. I'll give the short version:
A few summers ago, several of my X10 modules (actually, they were Insteon switches, running in X10 mode) suddenly stopped working, only at night.
I spent a few weeks trying to test everything in my house (flipping off every breaker,unplugging every device, etc), but found nothing.
My dad (an electrical engineer) even built a test rig to look for signal noise, but found nothing we could pinpoint.
Finally, he suggested it could be a neighbor's CFL, since it only happened at night, so something that was on only at night (like an outdoor CFL) was possible.
I tried the neighbor with the brightest porch lights first (who also happened to be a close friend). One by one, he unscrewed each bulb (they had been in service for years at that point - none were new), until we found the one that was causing the problem. When he screwed it back in, he could hear an audible buzzing coming from it.
I bought him a new pair of CFL bug bulbs for his porch, and the problem went away.
Afterward, my dad took the bad bulb (I asked the neighbor if I could have it), and tested it using the setup he had built. When he was closer to the bulb (maybe 20 feet of wiring), the noise was off the charts!
Both times I had a CFL take down my system (the story above, and one time before when it was one of my own), you could hear the bulb buzzing.
Maybe a neighbor on the same pole transformer has a failing CFL. It doesn't hurt to ask.