macbook quick freezes

i'm looking for some advice on an issue i'm having with my white c2d 2.4ghz macbook.

when scrolling in safari the page will freeze, then the spinning color wheel will pop up for a few seconds, then behave normally for a while.

clicking on something in any application will sometimes not register a couple times.

video playback will stop - picture pixelates - then recover and proceed.

these things happen maybe a couple times an hour.

i've recently installed a new 500gb 5400 rpm wd hd, but i think the issue existed before that.

it seems like when the macbook has to go to the harddrive to refill my 4gb of ram it stutters to catch up.

should this be the case? my processor speed, ram and hard drive shouldn't be taxed at all by basic activity like this, should they?

thanks - i'll do that tonight - right now i'm backing up.

i need my install disk for this test right? i'm not at home but my dad has the same macbook as mine so i should be able to get the disk.

the install disk didn't work - must be a bit different to mine.

but rember says the ram is good.

another weird thing is several times a day the optical drive does the "lets check for a disk thing" too