Ok, thanks for looking anyway, your help has been great!
I do have a 1GB stick of Crucial RAM that I've added but removing this hasn't cured it. I will keep it out until (if) I fix it anyway though, just so there is one less variable!
I'm unsure as to the exact whereabouts (lost it
)of my orginal iMac disk. i have tried a 10.4.0 disk that I've borrowed off a friend and it isn't booting up at all, showing that 'no' sign in place of the Apple after a few minutes of trying to access it.
I am thinking the next thing for me to do is to get hold of a Firewire HDD to mirror what is on the iMac now so I have a complete backup. Then I can try reinstalling Tiger and see if that helps or if not I can just wipe the internal hard drive a few times and send the iMac back to Apple. I'm going to order a Firewire disk now since I've been meaning to get one for the iBook for a while but in the meantime are there any other things I could try?
[edit] It's now been running fine for an hour so I am thinking that maybe the RAM was the issue and that it just was still a bit messed up the first time I tried after removing the RAM.
I'll wait another day before I declare case closed but it's looking hopeful *cue my Mac dying on me now...*
Thanks again!
[/edit]
Thanks once more for all your help! I'd be lost without it.