What part of my iMac 8,1 is failing?

Hi all!

I'm having more and more problems with my 24" iMac 8,1 with a nVidia 8800 GS.

It began some time ago while watching a movie stream and the whole system suddenly stopped. After a hard reset I was able to boot up to the login screen. But directly after logging in the graphics including the mouse pointer got "stuck" which was usually followed by a reset. When booting into Bootcamp Windows 7 the same thing happend.

I've read a lot about the nVidia problem, so I replaced the nVidia 8800 GS GPU with a customized nVidia 9600 GT board. After a complete reinstallation of Mavericks, everything looked peachy for some time.

Then I got problems when cold booting Mavericks. After the chime and the boot cycle with the spinning wheel is completed there seems to be a switch between graphics modes (the gray background color changes slighty). At this point I get graphics corruptions like a distorted Apple logo and a lot of flickering pixels. After some time the system restarts and the cycle begins anew. After maybe 6-10 cycles the system boots and I can start working. This usually does not happen when I directly boot Windows 7!

Now something new happend some days ago: after powering on the LCD and diagnostic LED #4 stayed off. Mavericks boots (I hear the ticks when changing the volume) and I can connect via remote desktop. After some more boot cycles the LCD comes back on and I can boot normally into Mavericks or Windows.

My feeling is that these problems are heat related because after some "up"-time (meaning failed boot cycles) I can boot and use the system without problems (even when stress testing).

Here is what I have tried on top of changing the GPU to narrow down the problem so far:

  1. Changed the RAM --> No change (although memtest86 found errors in the previousy installed modules)
  2. Installing the Nvidia Web Driver --> no change
  3. Installing different OSX versions --> no change

For my next step I want to replace the logic board, but with the dark LCD screen I'm not so sure afterall. Maybe the LCD or the PSU is faulty? So far I have not connected an external monitor because I don't have the adapter cable for that. ^_^;

So my main Questions are:

  1. Can a faulty LCD be the cause of an unstable system? or
  2. Can a faulty logic board lead to a dark LCD?

I'm willing to spend some more money on this machine. I know that this is dumb, but I feel quite attached to this computer, so here we go. But right now I'm not sure which part to replace next.

Maybe someone can give me more advice?

Sorry for the long post and the poor english!

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