What version of Mac OS is it running? Just curious.

In a pinch, try removing the PRAM battery. Also, try removing 1 stick of RAM at a time until stable.

Just a note: In order for Tech Tool Pro to boot on your iMac, you'll need to verify if the iMac has the right firmware update applied; I think it's version 4.1.8f4, or something like that. In the mean time, I'll see if I can get cracking on a bootable copy of TTP 4 for you.

73s de Phreakout. :rambo: