I had a problem like that one and was lucky to discover the source was a point to point wireless bridge someone had installed and thought they were being smart by using non standard channels like 5 or 8 or something to avoid interference..
I recognize I was very lucky to discover the source easily. I have heard of people renting or purchasing scanning gear and knocking on doors to try to find interference sources.. Very time and $ intensive.
Can you just use channel 11? That looks pretty clean for now.
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