I believe there is a misconception about
noise on power line; the so-called bad
products that cause X10 signal problems are
actually attenuating the signal, they are
not bad but rather meeting international RF
emission standards (CE).
I believe that 99% or more noise on power
line is actually signals being attenuate by
electrical devices.
High quality products like computer power
supply, high-end dimmer, TV etc. in order
to meet certain international standard they
use RF filters internally to reduce
emission, keep in mind the power line is
60HZ, X10 signal are 120KHZ, any power
supply with RF filters will attenuate or
virtually eliminate X10 signals.
Product that cause X10 signal attenuation
can be filtered on a one to one bases with
X10 power line noise filters.
For diagnostic a X10 signal meter is
essential, and of course make sure the
house was wired to standards, with proper
grounds and polarity.