Sorry to bump this, but the saga continues...
I took the iMac in to the Genius Bar a few weeks ago and left it overnight. They ran their diagnostics and told me the only failing component was the RAM (my 128 GB OWC kit.) I ordered another brand of RAM from Amazon (Timetec) and it seemed to resolve the problem for about a week, but then I got a couple kernel panics with it installed. OWC support won't let me return the RAM I bought from them, saying "under Magnuson-Moss Warranty Act the memory would have to be replaced three times for the same issue before it could be deemed a lemon and a refund offered." So they are putting me through the same series of troubleshooting steps I went through with them before I took it to the Apple Store. The latest was to restore some disk permissions via the internet recovery option. I did that with the Timetec RAM installed and it was okay today.
Tonight when I tried to reinstall my four OWC DIMMs (so I can return the Timetec kit) the computer wouldn't boot. I tried several combinations of slots and eventually got it to boot with only the second from the top slot and the bottom slot installed. When I tried all four DIMMs again it wouldn't boot, so I went back to the two slots mentioned above. It wouldn't boot on the first try, but I fiddled with them and it eventually booted, so at the moment I'm only running with 64GB installed.
Needless to say I'm very frustrated and wondering if I should figure out a way to cut my losses and get rid of this computer. It seems to me that there might be some bad RAM slots. Maybe I can get Apple to replace it, but at this point I feel like nobody can help me get to the bottom of this. Apple blames OWC, OWC blames Apple...
Suggestions welcome.