Dan,
I can't say with 100% certainty that the problem is the new monitor (I suppose I could plug the CM15a back into the outlet in question and see if the failure recurs, then unplug the monitor and see if it fixes the problem, but honestly I'm not that curious.)
Here's what happend:
1) Got a new monitor. It is plugged into a UPS on a different socket (but same circuit) as CM15a.
2) CM15a stopped working. In retrospect, I think it was "working" but many messages were not being received by modules. The earliest symptom was that my macros wouldn't work at all, or wouldn't work correctly (i.e. 2 of 3 lights would go on). I later realized many lights controlled by timers didn't work either.
3) Before I thought about the possibility that the new monitor was "responsible" there was a period of time where the monitor was OFF, though still plugged in, and I still had trouble (strange, but possible, I suppose)
4) Tried resetting CM15a by clearing, downloading, taking out batteries, no effect
5) Moved CM15a to another room (now disconnected from USB to computer) and it works fine (i.e. not faulty.)
The monitor is a 19 inch LCD. It wasn't the only thing I changed - I also removed a power supply for a pair of speakers from yet another outlet, but I doubt that made a difference.
I think the moral (for me at least) is: If X10 stops working, think about what is new in the vicinity, because anything can cause interference.
Paul