The cause of your problem is simple: Your "cache"
memory and "temporary internet files" are full.
If you are experiencing either: A) slugglish loading
and buffering, B) "choppy" videos which frequently
start/stop/start/stop, C) the YouTube dotted circle
looking like it might spin for ever, or D) videos that
will sometimes come to a sudden "stop" or freeze,
that is often a sign that your cache/memory is full
and cannot take anymore.
Exit out of YouTube (but not your browser) and do
one of the steps below. Afterwards, shutdown and
reboot. Not only will this "free up" your computer's
memory, everything will be "fresh", and load faster:
Internet Explorer 6:
→ Tools → Internet Options
→ Delete Cookies → OK
→ Delete Files → OK
→ Clear History → Yes
→ OK
Internet Explorer 7:
→ Tools → Internet Options → Delete...
→ Delete files → Yes
→ Delete cookies → OK
→ Delete history → OK
→ Close (browsing history window)
→ OK
Internet Explorer 8:
→ Tools → Delete Browsing History
[. ] Preserve Favorites (leave blank)
[x] Temporary Internet files
[x] Cookies
[x] History
→ Delete
Mozilla Firefox 3:
→ Tools → Clear Private Data
[x] Browsing History
[x] Download History
[x] Saved Form and Search History
[x] Cache
[x] Cookies
→ Clear Private Data Now
Mozilla Firefox 3.5/3.6:
→ Tools → Clear Recent History
Time range to clear: → Everything
▼ Details
[x] Browsing & Download History
[x] Saved Form and Search History
[x] Cookies
[x] Cache
→ Clear Now
Apple Safari 3:
→ Edit → Preferences
→ Security (tab) → Show Cookies
→ Remove All → Done
after that:
→ Safari → Empty Cache → Empty
after that:
→ History → Clear History
Google Chrome:
→ Tools (wrench) → Clear browsing data...
[x] Clear browsing history
[x] Clear download history
[x] Empty the cache
[x] Delete cookies
Clear data from this period: → Everything
→ Clear Browsing Data
DON'T FORGET TO SHUT DOWN AND REBOOT
AFTER DOING ONE OF THE ABOVE.
If you have other applications which are running at
the same time, those will slow down YouTube too.
If you have "Torrents", you might want to consider
closing or uninstalling them. A couple people who
chose me as "Best Answer" wrote that everything
worked great after they'd uninstalled their Torrents.
For some reason, simply closing them didn't work.
Also, "Adblock Plus", various Instant Messengers
and every "stupid video accelerator" under the sun
is known to interfere with YouTube.
Another thing you may want to consider is turning
off your anti-virus -- but only momentarily. As long
as you remain on YouTube (and don't scamper off
to other websites), you will be okay, because you
cannot get a virus from YouTube.