Advanced-N 6205 keeps dropping - Dell Latitude E6420

Hi there - first poster here !

I have a Dell Latitude E6420 with an Intel Centrino Advanced-N 6205 and periodically, the wireless keeps dropping and the eventvwr shows :

"Authentication failed due to incorrect security password (key) (WPA/WPA2)"

it also shows :

"Security mismatch with 11n. 11n is not in operation"

....but then it sorts itself out !!!

Router is fine as i have an IBM laptop that runs perfectly and has never dropped

Software versions show (according the Proset Help/About) as :

OS is running :

Windows XP Pro version 2002 / SP3

32 bit

Has anyone else experienced the same and if so, can you advise me how to resolve please as its doing my head in and seems pretty flaky !

Thanks in advance