I don't know about the lights locking up, but it may not be a problem with your IR543. Do you have any other way to control the X10 switches?

It could be:

1) The IR543 is on one phase and the switches are on another -- and you don't have a phase coupler/repeater. If you have any plug-in modules, try plugging them in to the same receptical the IR543 is on. If they work there, but not where they were, then this might be the problem.

2) You may have noise sources. I had one X10 switch that rarely responded to my IR543 -- either via remote or keypad on the IR543. It turns out an "always on" baby monitor which worked over the power lines was drowning the signal out -- it only effected one module.

I should say I have noticed that at times my IR543 misunderstands my Pronto -- I have "clean" codes downloaded here, but once in awhile it turns on the wrong device, but it USUALLY works correctly.