Which browser are you using? Some Firefox users have experienced problems with either the program itself (firefox.exe), or a component of the program (plugin-container.exe) hogging nearly all of the processing power, so that scrolling is slowed.
If you're using Firefox, check that you're using the latest version (5.0), as it seems rather less prone to the problem than Firefox 4.
To see if your browser (or, indeed, anything else) is hogging your processor, wait until you're experiencing scrolling problem. Them press Ctrl, Alt and Delete simultaneously. (If there's a delay before anything happens, that's already a sign of the CPU being 'clogged up' by an awkward program). When you see the display, click on the 'Processes' tab (if it's not already selected). Look down the 'CPU' column. Apart from 'System Idle Process', nothing there should have a very high number. (System Idle Process usually shows a percentage in the 90s, with everything else showing '00' or low numbers). Also look at 'Memory Usage'. Most things should show less than 20,000K but your browser might show around 200,000K. (Anything significantly higher is worrying).
If the problem can be identified as being related to your browser, try upgrading to the latest version (or reinstalling it), or switching to another browser. If not, let us know what you find.
Chris