I had the same problem a few months back. Heres what I did:
I changed the DNS as some suggested. turned off Proxy Server, MTU is set to automatic, UPnP is off
The 1st time I did that, it didnt work but after fishing around, I found out it was because my PS3 n another laptop was using the SAME IP address. I manually changed the IP address and set all the settings as I mentioned above and it I havent had a single problem since!