Every disc-based installed fails. New iMac. What's the culprit?

I have a relatively new 27" iMac. Received it in July 2011. Since the beginning, I've been having problems and AppleCare/Geniuses still haven't figured it out. I figured I would go to the community to get some thoughts.

My computer has an i7 and 16GB of Crucial RAM. Also a 256gb SSD. Running OSX 10.6.8.

Any time I use a CD or DVD software disc, it fails. This occurs with iWork, Adobe Creative Suite, even OSX itself. The first time I had this problem AppleCare walked me through reinstalling OSX. It failed and bricked my machine. I had to take it into the Apple Store. A Genius loaded a fresh image and I was back up and running. These images always work fine, that is

Since then, Apple has replced my optical drive and my SSD. Neither fixed the issue. Now, Apple says the likely culprit is my aftermarket RAM. I guess a test was done to isolate the issue. I didn't see this test, but a Genius told me he took out my 16GB and replaced it with 2GB of factory Apple RAM, and iWork installed fine on the DVD.

Since then, I've tested the RAM using Memtest. I ran it for about 24 hours straight, and got 0 errors. In the past, I've run some newer games like Team Fortress 2 with no issues. Nevertheless, Crucial agreed to RMA it. Hopefully, the problem will go away with the new RAM, but I have a sneaking suspicion it won't.

I should point out that all installers that are HDD/SSD based - OSX images, iWork images (both of these from the Apple store), Chrome, Filezilla, Steam, various games like TF2, software updates, other downloads, etc - work fine. No problems whatsoever.

So what's the culprit?

Have you ever had an issue like this?

Could the DIMM slots be bad, rather than the ram? Something with the logic board?