I've just installed 11.04 and now at random everything freezes. The keyboard responds but the only thing that seems to unlock it is a reisub. I never had this problem before and I've had this computer for almost a year. I'm using gnome 3 but it also happens when I'm using unity. Even if I don't have any programs running soon or later it freezes. I've seen some similar questions here like this and this but no solution.
Sounds like a sync issue (which I thought they'd already patched). Try this and see if it helps:
Grab the compiz settings manager like so:
Open it up and browse to the openGL plug-in.
Look for the "Sync to VBlank" option and uncheck it.
I had a similar problem. It turns out in my case that the r8169 networking module was unstable. Uninstalling the vanilla r8169 module Using the Realtek proprietary r8168 module solved this for me. I have no problems with stability now.