Have you tried going into the settings menu and shutting the extra windows? Or what about clearing the cache, history, etc? There is a limited amount of memory available for browsing and if these things aren't cleared up regularily the device doesn't work properly. (think of it as housekeeping/clutter control). You could also try the always popular "reboot", in other words shut the NC down completely and then power it up again, this clears out quite a bit also.
Or maybe, you need to clean out the "downloads" folder on your NC. Unfortunately the only way to do this is when you have your NC hooked up to your PC then navigate to the folder (not your picts folder, but there is an actual "my downloads" folder) and clear out all the downloads. Don't know if this would help, but was just a plausable theory to your issues...
(P.S. i have had issues in the past by not cleaning out the "downloads" folder on my NC, you would be amazed at what ends up in that folder just by looking around on the internet....)