I have exactly same problem. In my case it is 5100 AGN + broadcom 802.11n adapter (another notebook). It connects just fine, but when I start to copy file via network packet loss becomes 100% and it stops working at all. Windows 7 x64 both ends, latest drivers both ends. I have another important addition. When I reconnect broadcomm adapter, it does not help to restore connection. I have to reset it (broadcomm). Then it connects fine again (one single time). If I reconnect intel adapter - this does not help.
Can you open an issue for this to be fixed? Two cases makes an issue, right?
I submitted a support request at http://supportmail.intel.com/ on July 8. The next day I got the following email:
Thank you for contacting Intel Technical Support.
Please note that we are researching your inquiry and will get back to you as soon as we have additional information.
Please do not hesitate to contact us again if you need further assistance.
Thank you for your patience and understanding.
I haven't heard from them ever since. Maybe you shoud submit your own request, so that they know it's not a one-off issue.
P.S. I use an Ad Hoc network as a workaround, but at the sacrifice of speed (an ad hoc connection runs 11g, at least in my case).