Could be a faulty browser plug-in. Sometimes they get installed when installing other programs and they make the option to opt-out hard to notice. Did
you install any software recently? Try disabling all plug-ins to see if that works. Since your pages scroll slowly after the browser has been open
awhile, a bad plug-in with a memory leak could cause this. Could even be a virus. Wouldn't hurt to do a full scan just to be sure.

