I know you have had lots of suggestions, but, I thought I would inject my 2 cents. The booting to the screen with the disk animation is sticking in my mind. Have you yanked the cables to the HD and re-seated them? Also, I would try disconnecting the HD and then booting from floppy and playing a few games or something and then see if that problem goes away.
Hope this helps, I have a sneaking suspicion that the hard drive is to blame.
Thanks,
Mike
