My Dual 2.7 G5 does the same thing.
Worst of all I’d have to wait couple of hours before restarting… otherwise it won’t even reboot.
Plus my machine is so loud and the fan is almost always on even when I’m freezing in the Control Room.
I talked to Apple but unfortunately I don’t have AppleCare and they can’t do anything.
They told me to take it back to Fry’s (where I bought it from).
I do have Fry’s 2 year extended warranty… hopefully it will all be taken care of.