The solution to your problem actually depends on which version
of "flash" you have installed.
While the video is in regular mode (not "full screen"), right-click
on the video itself. From the drop-down menu, click "Settings..."
If you get 5 different icons on the very bottom row, you might be
in luck.
The first 2 icons should both show you a monitor. Click the one
that features a graphic on the bottom of its screen (probably the
first icon). You should thus get "Enable hardware acceleration".
Whatever the box shows you, click the opposite, and then click
on "Close". This seems to be the solution for quite a few people.
Although this advice seemingly contradicts Macromedia's own
website, you can always double-check for yourself. Here is the
URL address typed out in full, followed by its link:
www.macromedia.com / support / documentation / en / flashplayer / help / help01 . html
http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html
(Please don't start messing around with different browsers or
download utilities.)