Do you have a phase coupler or X10 signal repeater in the system? To get the X10 power line signals between the two incoming power lines. It could have failed.
Your description indicate you have a noise problem, coupling problem or a signal sucker.
Is the SC1200's power supply on the same AC circuit as the computing equipment or an UPS AC input?
Jeff has a great set of X10 troubleshooting tutorials. If you have not seen them yet.
http://jvde.us/x10_troubleshooting.htmI guess there is a remote possibility the SC1200's power supply has a bad X10 power line transmitter in it. As it supplies power to the SC1200 and sends the X10 commands back onto the power line. Do you have any X10 signal tester by chance?
X10 does sell a replacement XM19A but I would not jump on that bandwagon with out trying more tests.
Would you happen to have another X10 controller or transceiver you could try in the same location as the SC1200? To see if another X10 sender also has limited coverage.