Hello again,
Thanks for the responses everyone. I have been away the last few days so only got some time to spend investigating this issue last night. The good news is that I found out which extension is causing my problem (after sooo many reboots): it is one called "QuickTime PowerPlug".
According to the description of this extension it enables faster QuickTime performance when using a PowerPC processor. As far as I am aware the G3 is a PowerPC processor so I have no idea why this would fail
Anyway, with this extension disabled the system boots up and acts normally, except that I get a post boot dialog box warning me that the QuickTime VR extension failed to load. If I also disable that extension I get a different warning that the QuickTime VR Authoring extension failed to load (I think that is what it said). However, I cannot find an extension that matches that description to disable it.
I saw that there is a newer version of QuickTime available for OS 9.2.1 (v6.0.3) so I will try to locate a copy of that and see if updating my installation of QuickTime resolves the issues.
Another thing I may try putting the original graphics card back in, we'll see.
When I got the machine (bought it from ebay a few years ago) it came with an install of OS 8 or 9 (complete with previous owners files) that I played around with briefly before swapping the drive for a Linux install. I don't remember seeing any errors when I played with the previous Mac OS install so I may put the old drive back in and see what versions of OS and QuickTime seem to work and the state of the extensions as a point of reference.
GaryN: thanks for the link to the other thread, I will check it out.
Astroman: In fact I did originally try the 9.2.2 OS image but found the same behaviour. I reverted to the OS 9.2.1 image as (as far as I understand) the 9.2.2 image was put together by the community whereas the 9.2.1 image was the official release from Apple. I thought going with the official Apple image would be more likely to work or at least maybe narrow down the issue. No such luck though.
The machine has been working with with Linux and OpenBSD the last few years so I think the hardware should be okay but I may see if I can find a memory test to run.
RogerWilco: Regarding the image for 9.2.1, I downloaded it from the link in this thread:
http://macos9lives.com/smforum/index.php/topic,808.0.htmlThanks again for the help.
8bitSheep.