X10 is cheap for a reason.
It likes to do things it wasn't told to do because of noise (or your neighbor with HIS X10 stuff!)
I install this stuff "on the side", and publish very sophisticated home control software (freely available) for this purpose. Don't use X10 for anything that you really care about.
For things like my HVAC (heat, air conditioning, etc) I use a relay box that is hard-wired. It always works. The X10 works when it feels like it. You can radically increase its reliability by spending a bit of money on a phase coupler (and whole-house surge suppressor), both of which are installed at your main electrical panel, but even with those items it won't be 100% reliable.
Here's a link to the home control code - it runs my entire home, including the pool, hottub, heat, A/C, etc:
http://www.denninger.net/homedaemon.htm