can anybody tell me why, when i’ve had my laptop on for any length of time, it starts chugging?  the keyboard doesn’t respond to input without great delays, and sometimes even slide shows become flicker shows, and videos go herky jerky.  it seems to be a browser something as all i have to do is close firefox and the reopen it and it’s fixed for awhile.  i even reloaded the new firefox, but it’s still happening.  norton says i’m not viral.  what can it be?