MBP fails to connect to internet even though all signs are good

About 2 weeks ago my 2008 MBP running 10.6.8 spontaneously lost connection to the internet. Other devices on the same router/modem are not affected. Apple genius bar performed standard troubleshooting and hardware checks and all results were good. The suggested "cure" was a complete erase and re-installation of the OS - which I did. And it worked -- until yesterday when the same failure occurred. So, I erased and re-installed, again. Does anyone have any further insight into what may be happening and then of course how to fix this? Thanks.