One situation that can cause random operation is if there's a loose connection.

I just had a free-standing lamp that started to come on at apparently random intervals. The lamp was connected to a simple X10 light module (plug-in type). I eventually noticed that the light would come on it the lamp was touched or bumped which led me to the problem. The bulb was loose in it's socket which made an intermittant contact when the lamp bumped. That intermittant contact was seen by the X10 module as someone using the modules local control feature.

Regards,

--LB