what ISP are you using? I was going to suggest testing the hard drive, but I really don't think it would cause and issue like this.
Do you have this issue if you run a cable, opposed to wifi? Give chrome or opera a try and see if they fix the issue.
edit: also, try disabling zone alarm and/or AVG to see if the problem continues. I've tried using ZA a few times and never really cared for it much, it always felt a little too paranoid for lack of better words. If it turns out to be the problem (which i doubt, but you never know), give sygate personal firewall a try. Great 2 way firewall and it has a very small footprint