I... I've not added any new noise sources, but instead I've reduced noise by unplugging about everything in the house, trying to get the coupler working...
It is possible that the noise could be coming from outside your house.
I had that happen to me. A neighbor across the street (but on the same pole transformer) had a failing CFL bulb in one of his porch lights. When it was on, it produced enough noise to drown out my X10 signals, even WITH the XTB-IIR.
I was having trouble tracking the problem down, since it was only happening at night, and nothing in my house was changing between the time it would work and the time it wouldn't.
I finally asked the neighbor to turn off his outside lights for a minute, and the problem went away.
I bought him a pair of new CFL's.