This is going to be a non-tech answer I'm afraid but I've had similar with an old XP laptop. I used to be able to take down nearly any wifi network just by turning it on. It was the laptop's fault, it's ages ago now but I remember it always had problems since I stopped the 3rd party drivers and let Windows have control over the wifi settings. A thing of the past pretty much now but whatever it was doing, it used to stop everyone else getting on the network and it did that with 2 different customers. It worked fine at home, other devices would connect without problem, had a mind of its own outside!
1) have you changed passwords in case you've been compromised? </really-long-shot>
2) have you tried killing all other wifi devices bar one?
Often with this sort of problem, it's hard to see the wood for the trees and you're best off going right back to basic config and doing without any frilly settings and seeing how that works out with one device, add another and so on.
