Solved: N900 cause my router to reset after several min
 

I switched the WPA authentication from TKIP to AES and the router stops restarting,

This is a 3COM officeconnect router and my N900 was the only wifi client.

Maybe the auth implementation in the router had a memory leak that was caused by the key created on the n900. This didnt happens when I connected either laptops or other cellular phones (like Xpress 5800).

Well, at least I found a solution...