I have a Ubuntu 10.04 , I have recently been playing Minecraft by using the,"Minecraft.jar" After about 6-10 Minutes, My desktop suddenly freezes. Sometimes, I just spam Ctrl+ALT+* Sometimes it works and sometimes it doesn't. Then I would have to restart my computer, but would happen AGAIN later. Is there anyway to stop this from happening?
There was something (kind of) similar to your case. That case was (kind of or at least became a bit better) solved after uninstalling the game and Java package then reinstalling both.
You may also consider checking for any update that might have been recently released by your VGA vendor.