Same issue on both 10.04 and 10.10 (the upcoming beta).
Some random time after boot, the UI freezes in a weird way: I can still maximize/minimize windows, but not move them. Applet and other menus don't open, just get the "activated" color when clicked. All text input disappears, together with cursor, as if they can't get focus. It may happens several times on a boot, but usually it only happens once. It can happen immediately or after hours. Probably related, but not with 100% correlation (either may happen without the other, but often happens together) is that the volume control goes bonkers, it goes to max and just keeps on maxing, as if the button was being pressed repeatedly.
The solution to the volume thingy is to manually press Fn+F5, then it stops. For the freeze, the only thing I've found to work is to switch to a terminal with Ctrl-Alt-F1 then back with Ctrl-Alt-F7.
It's the same with or without desktop effects (compiz or metacity). I'm running only on the Nvidia card also, disabled the switching in BIOS since it's not supported by Ubuntu. Using the Nvidia drivers.
It could be a hardware problem, I guess, I don't use Windows with any regularity so can't really compare... but wanted to ask here if anyone has seen anything similar and have any clues?
Any other info you would like, I'd be happy to provide.
Thank you!