You say you've tried a different harddrive/cd-rom drive already? because this does sound like a bad drive (the cd-rom in particular), but it could also be that the cd you have is bad. did you burn it yourself, and did you check it before burning? i've had many problems in the past that I could have avoided if I only knew the power of the dark side.. I mean, checking the md5sum's of the ISO files I burned
Also, did you try swapping RAM modules for others, or different sockets?