Dear Rick,
thank you very much for your help (which Dell itself does not want to provide). I tried everything you suggested, but with no results regarding my initial problem. The former driver keeps interrupting my WLAN and not remembering access data. The new driver provided by Intel could not be installed properly and slows down the connection massively. An alternative driver from Dell's driver section I cannot find.
My computer is a Dell Latitude D820, a little slow by itself but working properly otherwise. I tried all the preference settings you were stating and used the Windows fix you were proposing. F2 wouldn't lead to a result, neither would Fn and F2. I think, sleep mode isn't a problem since the connection is lame right from the start. No ideas left, so maybe a new notebook as a last instance. No Dell, that's for sure!
Best regards!