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Wireless b/g/n (n doesn't have to, as it has two frequencies) and Bluetooth both use the radio same frequency spectrum (2.4GHz). Anytime you use the same spectrum, you can get interference (and thus microwaves, cordless landline phones, etc, that produce 2.4GHz spectrum can also cause interference).

Usually it isn't a problem between bluetooth and Wifi, but if you're having problems, there are two major options:

1) Move your wireless network over to 5GHz. If you have 802.11n, you can pull your wifi off of 2.4 and move it over to 5.0, which should resolve the issue. You have to make the change at the router.

2) Wifi also has a channel which can be set (also in the router). The channel is just a sort of strip within the 2.4GHz band -- not a completely different frequency. Often if you try a different channel, the problem can be managed that way, too. You can do this regardless of whether you have an 802.11n router or a G-class one.

OTOH it's odd that it would be bandwidth constained in the sense you describe -- it's odd it works fine until you try to transfer a big file. I've never personally seen that before.
 
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