Vista - Asus N51VC with Windows Vista disconnects randomly from Internet.

Hello,

I have just bought an Asus N50VC with Windows Vista and this laptop disconnects randomly from Internet. It is not that it lose the Wifi signal , but it looks like the laptop stops recieving data after a while; Firefox (or IE) tries to get the web page and eventualy displays an empty page (white, without error message). Then, I have to disconnect my laptop from the Wifi signal and to re-connect it to the same signal so that it works again.
I have in parallel two other laptops (Windows XP and Windows Server 2003) and when Internet does not work on my Asus N51VC it works without problem on two other PCs.

Here is all I have already try, unsuccessfully:
- I have updated the Wifi driver
- I have installed the SP2 of Vista
- I have uninstalled all Antiviruses, Firewalls, Defenders ...
- I have forced my computer no to deactivate my wifi card (even in case of energy saving)
- I have tried with or without fixed IP with or without the DNS recommended by my ISP
- I have reinstalled the initial image of the OS such as supplied by the manufacturer

I do not know any more what I can do; does somebody would have any idea?

Thank you