It sounds like a noise problem or a signal loss. You say you didn't add any new X10 stuff but what about any new electrical devices such as TV's and such? Have you moved any electronics around to differant outlets? I had a light you could send an On but not an Off. Was due to a power wart for a GameCube.
(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)----KDR