I have had AM486's turn themselves off instantly when trying to turn on switching power supplies too. I switched all my LV lighting near the house to 12V LEDs. I dropped my power consumption by 80 to 90%. I have strayed away from X10 for this application and have used Intermatics Digital Timers because of this. I am doing far rear yard LV and really want to use the X10 so I can control them via the PalmPad. I am trying to avoid using a X10 control relay module wired to relays to do it. I ran across this thread while looking for solutions
On a side note, CFLs that are turned on in circuit near the X10 module will render the X10 deaf too. I had an AM486 on a Christmas tree. It came on every night via Active Home, but not always off. Then I'd try with the PalmPad, and sometimes it would work. Finally I realized it was when I had the Porch lights on (CFLs / manual switch), the AM486 was deaf. It seemed totally random, but it was just my randomness of when I was turning the porch lights on and off. Even the signal amps couldn't over come this. I may try switching brands of CFLs.
I have also installed a bunch of X10 XPPF in front of equipment with switching power supplies like Computers, TVs that are not on UPSs or Power filtering equipment like APC's H15 to keep my signals from being eaten and clean and I've installed the X10 XPCR. Unfortunately I can't put XPPFs in front of wall switched permanently mounted wall fixtures.