Tuiceman is correct, there are issues with any of the various "flavors" of home automation or remote control.
X10 was designed in late 70's, early 80's..."eons" before the first switching power supply. Now days switching power supplies are becoming so cheap to produce, many "wall-wart" plug power modules now contain switching power supplies rather than the ubiquitous transformer. Unfortunately the "noise" generated by switching power supplies can mask and swamp X10 signals.
Equally unfortunately, there is no 5 cent silver bullet.
Have you explored Insteon, or Z-Wave, etc? The newer technology relying more on RF is more resistant to noise, AND more expensive.
If you want to keep your X10, figure on spending a little more money.
You seem to have a handle on what X10 nemesis are (line noise and adequate phase coupling).
1. I would dump the passive phase coupler and get an amplifer coupler. The stronger the X10 signal, the better the noise immunity.
2. Buy an X10 signal meter and use it to survey your home. The signal meter will identify noise sources as readily as X10 signal levels.
3. Buy or make filters for only the most noisy offenders (suspect any big screen TVs less than 15 years old, VCRS, DVDs, cell phone chargers, and any wall wart that is so light in weight it makes you think it is empty - switching power supplies are much lighter than the old transformer supplies, so any electronic gear that "feels light").
I have a techno geek home loaded with switching power supplies AND X10 (100+ modules and I just ordered more). Sure there are times I or the home automation system sends commands that do not get through but X10 offers too many ad vatages to ignore.
In the 1980' my wife was frustrated with X10 also and wanted to "put it all in the closet" because of undependability....until I blew up my old home controller and had to order parts. for two weeks nothing took place automatically, we had to actually go to the switch to turn on lights, we had to roam the house every night to turn out lights before going to bed, we came home to a hot house (Florida) and a dark house, etc. etc. etc. Exactly two weeks into the three week wait for parts my wife asked me "when in the H are you getting those parts so we can get back to normal ?!" ....have not heard a negative peep since.