Some thoughts:

Post your motherboard, cpu, power supply specs, video card model so people with them can offer insight.

Check which screensaver Unity is trying to use, on 11.04 it should just be a blank screen, but if it's trying to use something else it could be locking up the driver when it kicks in.

Some people have said that leaving a mouse plugged in can cause problems if it is constantly seeking it's position on the screen. Make sure the mouse isn't near the edge of the screen or try unplugging it.

Are you setting fan speed and checking temps?

What power supply are you using and what is it plugged into? You should probably have a minimum 650 watts for this machine.