Sounds like a lemon. My Cube 450 runs flawlessly. I did have it sent in though because it liked to turn on by itself. Mine would also freeze on bootup but to fix this they replaced the AGP bracket.
After it came back from repair it has hummed along nicely ever since. If Apple can't fix the Cube within atleast 3 tries, they should replace it for you.
They replaced my PB 550 after two attempts to fix the AC port and various other issues which took over a month in time.
I basically told them, look, I'm a student and can't afford this downtime, this is unacceptable. They'll tell you at first all they can do is keep trying to fix it but don't believe them.
If it is still under warranty, you just need to be firm and let Apple know this is unacceptable and ask what they are going to do for you to rectify the issue. Let them know that continually sending it in for repair is no longer an option.
Chances are they will replace it with something new.
But don't go calling them up and telling them that you heard of someone getting their Mac replaced. Just make them understand how all the problems with the Cube are affecting you personally.