The lights have never gone off by themselves -- god, that would be even worse, wouldn't it?!
I still think replacing everything X10 with standard switches would be the way to go.
I think you're right. I see a trip to Home Depot in our very near future. It's funny -- when it first started happening, it was kind of fun (in a slightly scary way) to think of past residents of the house messing with us, but as soon as it became clear that it was an automation issue, I just wanted it done with. So, friendly ghosts = OK, but lighting/wiring mysteries I can't solve = not OK. If nothing else, I've learned something about myself.
I'd love to put one of those X10 switches on my test bench and test it in on an isolated circuit but I imagine at this point you're more inclined to want to have a hammer smashing party.
Ha! Nah, no hard feelings toward the switches. You've been so helpful with all this -- if you're serious about wanting the switches, just say the word and I'll send them to you! I don't need to smash them, but I certainly don't want to keep them around either.