When I upgraded from Linux Mint version 6 "Felicia" which is based on Ubuntu 8.10 to version 7 "Gloria" based on Ubuntu 9.04 my video performance started suffering badly. Worst of all, I couldn't watch videos using any of the applications installed vlc, miro, gmplayer, mythTV, Totem.

To see the kind of errors I was getting refer to my bug report

What worked for me was reverting the Jaunty Xorg intel driver to 2.4 as explained in this link