The fact that the maxi controller works is good -- but the IR543 not have as strong a signal and may be drowned out or pick up interference. I would recommend determining for once and all if you have a signal to noise problem or a faulty IR543.
If the IR543 and X10 switches are all on the same circuit, you don't need a coupler.
1) Are any of your X10 switches plug-in modules?
2) Does the baby monitor work over the power lines or is it true RF -- i.e. it has a transmit and receive antenna? If it works over the power line, then yes, it can be causing your problem. Turn it off, and see if the IR543 works. If it is RF, or doesn't fix the problem when off, then try the following:
If (1) is "yes", then take the plug-in module to another room in the house (or even preferably to someone else's house). Plug them both in to the same wall outlet and see if the IR543 works properly. If it does not, then you have a faulty IR543.
If (1) is "no", then turn _everything_ in the room off and _only_ plug in the IR543 and one component (preferably a non-electronic component like an incandescent light). If there are any appliances that are "always on" turn them off too (refrigerators, air conditioners, fans, well pumps, boilers, etc). The easiest way to do this is to turn off all the breakers in your house except for the circuit your IR543/X10 switches are on. If the IR543 still does not work, it is faulty. If the IR543 now works, then turn the circuits and components back on one by one and see when the IR543 stops working. This will tell you what circuit/component is causing the problem.
Some additional questions to answer:
Is the house code wheel set properly on the switches? What about the device numbers? Remember you can only control one house code the IR543 (and it must be set the same on all the switches as on the IR543). The IR543 will only control devices 1-4 or 5-8 depending on how the rocker button is set. So make sure the devices you are controlling are set to devices 1-8 to use the IR543 manually (you can control all 16 units with the pronto).
