How fast is your computer's processor? How much RAM do you have? You might be able to solve it just by installing an extra 256 or 512 MB of RAM. Do you have torrents running as well when this is happening? I would recommend uTorrent if you don't use it already. It uses a tiny amount of system resources (it's not really "u" Torrent, it's actually micro-Torrent, the "u" is the symbol for a micron ). uTorrent made a HUGE difference for me, and I have a relatively slower PC. I used to have problems playing back video as well when I was running Azureus. Then I switched to uTorrent, and have no problems playing video. It might just be that your computer isn't powerful enough to handle multiple apps running at once that use a lot of resources. Does it happen even after a restart?

A couple additional things you could try:
-see if your hard drive needs to be defragmented (start > accessories > system tools > disk defragmenter)
-scan your computer with anti virus and anti spyware programs. Viruses and/or spyware can really slow down your PC. AVG is excellent and free, and offers both anti-virus and anti-spyware programs. Get it here:
http://free.grisoft.com/

Finally, where do you go to play @ TTD teabaggins?