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Mine was a conflict between 825 and Intel 5100 that used to force the router to work in 20MHz even though Fat Channel was disabled.The problem was that 825 couldn't switch to 40MHz after that so I had to reboot.I didn't use 5100 anymore and seems it's working all the time in 40MHz
This is another story. It may happen due to long distance, signal conflict or maybe your WLAN chip got weak.Try with another device or router.