You are running the x64 version of W7 since you have so much memory?
Otherwise, what is your setup. Do you go through a router? Any logs around the time of the lockups by any anti-virus (AVG in your case) or a firewall? Do the delays happen on any certain sites, or do you see any icons or messages in IE on the bottom of the window?
During these lockups, can you ping an address on the net?
Do you show DNS set up on your system?