My Powermac G4 MDD has worked fine, but now after the white apple boot screen (the one with the grey apple logo), the screen suddenly goes black. I have tried resetting the PRAM (cmd-alt-p-r), but this doesn't work. I don't understand why this is happening. The Specs: Powermac G4 MDD Dual 867MHz CPU 512MB RAM 2x 1TB Hard Drives I really need this to work! Please help!