Problem w/ two wireless PC's at ones

Have a MN-700, MN-720 and MN-730. WPA enabled, same SSID,
same workgroup, and latest firmware/software. When I
connect to Internet both wireless adapters work fine, but
if I start to download with one computer the other one
drops the connection. The network icon in the system tray
says (on the dropped PC) "Signal Strength: Excellent" but
I can't connect to the base station/Internet unless using
a wire or reseting the base station (does not help to re-
boot the PC or enter the WPA key again). How can I stay
connected and download/upload on one computer without
dropping the other connection??