troubleshooting x10 can be a pain. I agree that you should first change housecodes, then also from your activehome menu with the cm11a plugged in to your computer--go to tools, find other computers--and clear or reset this function. if a housecode change stops your problem then you can check this "find other computers" which should be labeled "find other x10 users" to see if someone is infact using this housecode.
if that doesn't clear it up, you might also try changing brand of lightbulbs. cheap x10 switches with 2 wires (usually black and blue or red) bleed power thru the bulb filament to complete the circuit to "listen" for an x10 signal. I've heard that some incandescent filaments can be rather nasty and cause problematic switch behavior
if the problem still persists, you probably should consider using a more expensive switch, one with not only a hot wire (black) and a load hot wire to the bulb (blue or red) but also a white neutral wire (if your switch box has white (neutral) wires in the box). these switches complete the x10 circuit without having to go thru the bulb filament to neutral. this may help the most.
if that doesn't do it, then I would check for noisy applications on this circuit (roof mounted ventilator fans set on thermostat, etc and if there is one, perhaps an inline-noise block may solve your problem.