obviously, this is not hardware problem.
this is how I fixed my problem on Sony VAIO notebook with intel WIFI link 5100agn
reading through this post from beginning, give me some idea below
1. some router did not implement "Power save polling" correctly
Intel� Wi-Fi Products — Power save polling (PSP) causes connection issues with access points.
2. some router have WMM problem, upgrade router firmware or disable WMM on intel wifi 5100 fix problem
explain by Wickham http://communities.intel.com/message/153318#153318
3. some power management setting cause problem with intel wifi 5100
http://communities.intel.com/message/153458#153458
The combination of above solution + update latest driver from intel website
then run "netsh winsock reset" fixed my problem.
maybe because some update between microsoft and hardware driver causing problem in TCP/IP stack
and run "netsh winsock reset" fix those problem.