First try changing the house code. You may have neighbors that are now using it too and everyone is using A1 as their code.

Thanks groundrod. I already tried that. I changed one of the switches thinking it may have been a bad switch and at the same time changed the code on that switch. That afternoon, the fan came on again just like the day before. This is why this nonsense drives me crazy.

Do you have noise filters on all your flat screen TV's and other electronics?
Compact fluorescents also cause many problems.
Neither of these were around when X10 was developed.
XPPF will cure 99% of the problem in most cases. Switching to
non-flourescent lighting may be necessary if you can't find a brand
that's compatible with your installation.

Here is a link that might be useful: XPPF

While fun for hobbyists, X10 is considered too undependable to control any home security/monitoring system. Just a thought.

Thanks saltcedar. It looks like that might be a solution. Any idea on how I figure out which appliance(s) are the issue?

Unplug each one and see if the problem's resolved.
It can take quite a while and some persistence. As a
rule I gang all my electronics as much as possible and
plug a power strip into a filter. Then I know everything
on that strip is OK.

I used X10 in my previous home. It quickly solved some poor wiring issues (bad switch locations) but started to act up when my neighbors started using it too. I also found that computers interfered with the X10 signals. I had lights mysteriously turning on and off and some lights would not work at all when computers were on.

I switched over to Insteon technology in the new house. Insteon is backward compatible with X10, but more more reliable when you use Insteon devices. The Insteon devices use a dual network (over wire and wireless) to make communications more reliable. Each Insteon device is a wireless and over wire transceiver - so the wireless & over wire network created as part of your switch & controller deployment.

Like all DIY products, Insteon has it's own quirks but I find them easy to work around. If you use this technology, get the Houselinc II software which allows you to set the network up from your PC. An Insteon network is much harder to set up (very hard if you try it from the front panels of the switches and contollers) - it is not like X10 - so definately use the software.

You could start out by only replacing the X10 devices that are having problems with Insteon devices and leave the rest of your X10 devices in place or incrementally replace.

You can find product info at www.smarthome.com or www.smartlabsinc.com.

There is also a user forum at http://www.smarthome.com/forum/forum.asp?forum_id=9.