Continuing internet frustrations.

We have two mid 2009 MBP 13" at home. Both have upgraded HDDs. I also have a now 1st generation Ipad. My wife's MBP and my Ipad at fast on without a problem on our home network. My MBP, however, is a different story. After surfing the web for about a half hour, it is as if something builds up and I am unable to effectively load certain websites any longer - which ones are often random. Once I restart the computer everything speeds up and loads well for a little while. I've tried cleaning caches, did the permissions thing, and everything but standing on my head. I even - just last night - replaced the hard drive thinking that might be the issue. I read somewhere that once you replace the hard drive in a unix based laptop, the heads continually park or something like that. But, then there's my wife's laptop with a replaced HDD and no internet problems on the same network. For the life of me, I cannot figure this one out. Does anyone have an idea of what might be causing my trouble with loading webpages after surfing for a relatively short time? Frequently having to restart is getting pretty old pretty quickly. Thanks.

Oh, I should say that it doesn't seem to matter which browser I use. Firefox is my preference, but it happens on all of them.