I finished off my basement a few years ago. I thought it would be cool to add a little high tech flash, so I put all of the basement lighting on X10. It was nice because I only had to run one wire to a bank of X10 dimmer switches that lets me adjust all the lights from a single spot. Cool.
Until a few months ago.
Now my lights turn on and off by themselves, and I don’t know how to diagnose the problem. I cannot think of any new electronic devices in my home…my assumption is that some signal is coming in from outside the house. It started with one particular light, then over the subsequent weeks the other lights in the basement started turning on and off by themselves. A light may switch on or off every 15-30 minutes now.
Maybe my house is haunted. Nah, that’s a stupid theory.
Do I need a Whole House Blocking Coupler? I’m reluctant to spend $75 and rip into my main breaker box based on a hunch. Is there any way to know what is causing this problem before I start buying new devices?
Diagnostics
Do you have a computer-based X10 controller? Something that's listening to your power lines and logging the commands it sees. This would be helpful for determining if you're getting actual X10 commands from another source (like your neighbor) or if your switches have gone nuts and are just turning on without being told to do so.
I had the same thing..
In the apartment that I used to live in, this happened quite often on one of my keyfobbed lamps.
I think it was just someone nearby with a signal that hit the same frequencies occassionally because since we moved, it hasn't happened since.
And the problem is...
You mean you didn't want that to happen? Just tell folks it's another hi-tech gadget that you control with your brainwaves. Wait, they might not think too highly of your brainwaves.
Possible causes and how to distinguish them
There's a couple of things that could be going on here. First, if you're using X10 lamp modules with fluorescent or compact fluorescent bulbs then the trickle of current through the bulb's ballast will eventually cause the X10 lamp module to 'think' that a connected light has had its switch turned on and the module will then switch on.
However, this does not account for the lights turning off again. So it's more likely that the X10 modules are hearing instructions on the wire. They'll either be real data, or random noise that occasionally looks like real data. Try changing the house code used by some (or all) of the modules (and of course the control unit) -- if your system is picing up signals from a neighbor, and that neighbor does not have a fully tricked-out system then you'll likely be able to find a house code that's not in use.
Finally, Noise is a trickier one -- you may not have installed new gear, but a neighbor may have done. If this problem occurs only during particular times of day then that's an indication that some new piece of kit is now in use. If it's continuous then the new kit could be something like an A/C or fridge that operates very regularly through the day, and your best recourse will be a filter on the lines entering your house. I've heard good things about the smarthome blocking coupler you mention in your post but I don't have any personal experience with it.