I got an extension cord and ran the interface to a plug on the other side of the room.
Sounds like your PC or UPS or peripheral is the culprit. A special X10 noise filter ($10-$25) between the appliance and the wall socket ought to take care of that. They come in 5, 10, and 15 Amp models from X-10 or Smarthome.
However don't discount the possibility that the socket on the other side of the room is on the opposite phase of the AC line and that's what makes the difference. If you have an AC voltmeter (or a $3 neon bulb voltage tester), measure between the hot pin of the extension cord and the hot pin of the socket near your computer. 230V (or lit neon bulb) indicates opposite phases. CAREFUL! don't get zapped!.
One further possibility is that you've had the CM15A plugged into a plug strip which unbeknownst to you contains a line filter and/or surge protector. Either of these is likely to suck up X10 signals.
Or if your tired of the whole hassle and won't trip over the extension cord, just enjoy your new toy.