It sounds like you have identified the problem, however, if not, one culprit not mentioned here is dimmer switches. As these usually cause interference on low frequencies using AM, it is probably not the source of your problem.
A problem I have in my unique location is being right on the edge of the main telephone microwave path in town. I'm less than a mile from the source and front end overload can result even to triple conversion scanners. It is very random and only occurs ocassionally, but once the receiver lets it through it can last longer than my patience. It doesn't help that I have a pre-amplifier that works at the antenna either, but without it I can't listen to 800 MHz repeaters located 60 miles away in my mountainous location.
The discussion here reminds me of how inadequate the Part 15 regulations seem to be. I've taken some fairly expensive commercial handheld radios into stores where the computer/cash registers put out enough RF to open the squelch from ten feet away. Annoying to say the least. I've noticed on my amateur handhelds frequencies around 146 MHz seem to pickup more of this interference than those above that. Can this be by design? Reminds me of the BPL situation, broadband over powerline, where the commisioners of the FCC concluded that most of the interference problems would affect amateur radio, and thus did not think it an important one.
