I have an X10 wall switch (PLW01) that often turns on by itself. None of my other X10 devices do this. It just controls an incandescent bulb.
I have done the following:
1. Set up a test lamp to the same address; the test lamp never comes on when the switch does.
2. Changed the address of the wall switch from A6 to A1, but the problem persists.
3. Noted that the switch's coming on does not seem to be related to the AC or pool pump on the same breaker box.
4. Using Indigo confirmed that no X10 signal is being sent to the wall switch.
What's the best solution?
It sounds like you've done a good job of eliminating possible causes. Since you've verified that no X10 signals are being transmitted during the "occurrence", I would guess that you have a local load problem on that particular circuit.
Somewhere on that circuit you have a device that is spiking the line during turn-on or off. This is in turn coupling into the PIC in the PLW01 and fooling it into turning on. Possible culprits include fast start fluorescents, motor loads (devices with high capacitive or inductive loads). If you perform an inspection of the devices connected to this circuit, I suspect you'll find one of these.
How do you fix this? Well that's a bit tougher.
1) It's possible that you have a problem with the circuit itself. Try checking for loose connections throughout the circuit (including the connection to the breaker and neutral bar at the box).
2) You may have a issue with a connected device (tube going out in a fluorescent).
3) You may be at the end of a long electrical circuit where the line impedance back to the panel is high (multiple series connections through outlets). This one is tough. What you need to do is lower the apparent impedance of the circuit. A filter may help, but the type will depend on the type of connected load (capacitive or inductive).
Try isolating the problem to a particular device on the circuit and get back with us,