I'm having the same problem as with quite a few people I see from searching X10's website. I did come across 1 post which I am going to try tonight. Try setting your floodlights further away from your motion detector (i.e. spread them further apart or maybe up or down) Apparently the heat from the lights can cause the sensor to trip as they are heat sensors not actual motion detectors.
I had these hooked up to my previous house and everything ran fine for over 2 years, but I do admit I had the lights pointing directly down while the camera was actually facing directly straight out, Well good luck, I will try it tonight and report back in a few days. If anyone else trys please let us all know before people start going crazy around here....lol
Cheers
Jason