I am running Windows 7 64-bit and I have been running into to some problems with my wireless adapter.
I have the D-Link Wireless N Gaming router with the D-Link DWA-552 Wireless Extreme N adapter for my desktop.
When I start my computer up, it connect just fine and I am at about 300mbps. After a few minutes (sometimes seconds), the connection immediately drops to 54mbps, and sometimes just starts disconnecting then reconnecting and it does this over and over. The connection becomes unbearably slow.
I have updated my adapter, restarted the router, changed WiFi channels, etc. with no luck. MY connection wizard tells me that the reception is anywhere from 95-100% connectivity, so I don't think it is a range issue.
What type of area do you live in, house, apartment building? It sounds like you are getting a huge amount of interference. Have you tried this sitting very close to the router, do you get the same thing? Does your software tell you the db signal strength? If it does, what is it?
Have you tried power-cycling the router, does that clear things up for you?