Weird Problem - can only see Mac in "Share my Screen"

Question

I've been having a strange problem that I think might have something to do with OpenGL, but I'm not sure. I think it might be because I've gotten some error messages that said that OpenGL wasn't available or working properly.
When I boot my mac, I have a hugely corrupted screen (gray with green stripes) which doesn't change at all. However, I can hear things starting up, and when I use "Share my Screen" from my MacBook, I can see everything, and the computer runs just fine.
I've tried attaching a different monitor; it looks exactly the same, and both monitors run perfectly when attached to other computers.
I tried taking out all the cards and things, wiping everything down, and putting them back. No change.
I tried booting in Safe Mode. It never actually booted. (I left it to try for about 20 hours before I gave up.)
I've tried resetting the PRAM, and that didn't help. (Before that, I had scattered green dots, and a fractured apple logo, with the widget that showed the computer was loading, until the point where the screen should turn blue, when it would freeze. Now I just have green stripes.)
I've tried to reinstall the original system, but when I attempt to boot from the CD, I get a kernal panic. This is true, no matter what CD I attempt to boot from.
I've opened the computer as a Firewire Drive (using an older Macbook) to change the system from there; but you can only install the stystem on the boot drive from the disks I have. While I was running it like this, the Firewire icon showed up just fine on my monitor, white on the gray with green stripes.
I've tried upgrading to Lion (I was using Snow Leopard) and was able to do it, but that didn't solve anything.
I don't appear to have the Lion reboot partition. At least, if I do, I don't know how to find it. The keyboard shortcuts don't do anything as far as I can tell. Since I cannot actually see anything on the screen until the comptuer is fully booted, so it might be there, but invisible.
Unfortunately, I didn't try the old install disks until after the Lion upgrade. Now, if I try to just install from the disk (using Share my Screen, so I can see what I'm doing) I'm told that I can't.
When I run Disk Utility, it says everything is fine.
The computer is due to be replaced in October anyway. Should I just give up and get the new one now? Or can any of you think of anything else that I might be able to try? Everything is totally backed up, so if I need to wipe my disks, that's not a problem.
The computer is a
     Mac Pro
     2 x 3 GHz Quad-Core Intel Xeon processor
     4 GB 800 MHz DDR2 FB-DIMM memory
     NVIDIA GeForce 8800 GT 512MB Graphics card
     OS 10.7.4
Thanks for any help you can give me. I'm at my wits end!
Robin

Answer 1

Hi Robin,
The no boot in Safe Mode is a big clue, generally a serious Hard Drive problem, but can be others.
Are you saying the blotched screen showed up fine in Screen Sharing? If so the problem is downwind of the Video Card/VRAM, & you seem to have eliminated the Cable & Monitor, so I'd quess Logic Board most likely.
On the HDD problem, try Zeroing the drive while it's in Target Mode...
1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
   2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
      *Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.*
   3. Click the Erase tab.
   4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
   5. Select that iMac volume.
   6. Highlight the drive, select Partition Tab, then Format type... MacOS Extended Journalled, select the Security Options button, choose Zero Out Data one pass, Erase... after completion do a new install... after completion hopefully you'll be able to get a good install.