Powermac G4 bootup problems

Hello there, i am relativeley new to Macs and my Powermac G4 (AGP graphics) previously ran OS9. I did a firmware update and then upgraded to OSx 10.4 and this is where my problems began.

The machine sometimes boots up (OSx) without a problem. Other times, it will not. It is hit or miss with no apprent pattern. Here are the symptoms.

Upon bootup, you get the chime. The screen will then turn grey and a square will appear in the centre of the screen. If all goes well, a revolving baseball will appear and the machine will bootup normally.

But here's what happenes when it doesn't bootup:the grey square turns black and the machine shuts down. If you reboot, it may bootup normally or fail to boot up.

I have tried substitutiing ram and also a different hard drive. I have even cloned a hard drive which i have used in other machines and the result is the same.

In case it helps, i'm running a 120 Gb hard drive, 756 Mb of Ram and the CPU is 350 Mhz. Any help would be appreciated - it's driving me nuts!

Thanks and best regards,

Phil.