Intermittent crashes on boot

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A friend just got a second-hand G4 (Sawtooth G4/400) with OS 9.1, and it intermittently crashes during boot. It always gives the smiling Mac, and always gets to the Welcome to Mac OS screen. Sometimes it crashes there, but most of the time it makes it to the Starting Up screen. It often crashes once the progress bar moves about 1 cm along - the mouse stops moving and the system completely freezes. Sometimes it makes it past that point, in which case it usually boots the rest of the way successfully.

So far, we've tried changing the memory, booting with extensions off, booting from an OS 9 CD, resetting PRAM, resetting Open Firmware (reset-nvram, set-defaults, reset-all), and resetting the PMU. The hard drive makes some interesting noises, but they might be normal (it's an old drive). We haven't yet tried booting from CD with the HD disconnected.

Any ideas? He's bringing his Mac back on Saturday, so it'd help if I have a list of suggestions to try by then

Edit: Sometimes when it crashes, it comes up with a white box with nothing in it (it looks to be the same size and style as the old 'Welcome to Macintosh' box from System 7).

 

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Update time

He brought it back around today, so we tried a few things.

First off, booting from CD with the HD disconnected doesn't help. So the problem isn't the hard drive.

We installed the latest (I think) firmware update (4.2.8), so the computer can now support OS X. The 10.3.5 CD boots fine, but it kernel panics a short way into the installation (when you're choosing options - it hasn't actually installed anything by this point).

Tried booting from the TechTool Pro CD, but it crashed on boot. No message, it just spun the little spinner thing indefinitely (and the CD drive wasn't doing anything).

Hopefully this new information will help with diagnosis

 

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Another update: I replaced the battery, which didn't help.