The fact that the G4 is freezing when booted off the install disk indicates that it is probably more than (or not at all related to) a HD failure. Do you have the hardware test disk for this machine? If so, try to run that.
http://docs.info.apple.com/article.html?artnum=31241You can also try to remove all the RAM except for the original Apple RAM, if you can identify it, and then, if it works, add the RAM back one stick at a time. Or just try testing the RAM one stick at a time - when/if you find one that works, add the next one and reboot, and so on.
However, you could also be looking at a more serious hardware failure such as the logic board, in which case the repair may well cost more than the machine itself is worth.
Let us know how things go.
Thanks for this - and apologies for my late reply, I've been pretty busy and so have let the problem sit. Your suggestion to try the Hardware Test disk is worth a look - and thanks for the link to the Apple site.
I'll let you know how it goes - meanwhile the machine does start and maintains stability for a while.
Thanks again.