I guess you were right. My problems with NP900X4C are not over. After Samsung service replaced the dreadful Centrino Advanced-N 6235, there ware no problems simple surfing Internet (using 802.11n). Streaming (for example youtube) was much better than before. I was so sure the problem has gone.
But yesterday It disconnected twice. Today I used the ultrabook with router which caused the least trouble before, but there was again a single connection dropout (with wi-fi icon with * as usually). I managed to test large file transfer. It causes the wi-fi to disconnect (yellow exclamation mark). Then again I had to manually turn the wi-fi off and on.
I'll give it chance tomorrow again. If it persists I'll give it back for service again. But then what? I see that Samsung keeps selling NP900X4C-AO2PL and newer NP900X4C-A03PL, but both have the same Intel® Centrino® Advanced-N 6235. Don't they know the problem or is it solved?
An option would be replace Centrino and lose guarantee - risky. Is the wi-fi card removable or soldered in ultrabooks?
My opinion is manufacturers should let customers chose: money back or replacing wi-fi card with something else than this rubbish.
Do you know any other good ultrabook suitable for working (which means - with at least 15 inches screen) without hopeless Centrino Advanced-N 6235?