Try 1 and 3 just to see. Usually if you place the memory in the correct place and they are still in single channel its a the CPU that can be the problem. Sometimes thermal past or oil can gets stuck on one of the CPU pins and cause it not to work correctly.
Try pressing the ram dimms a little more on to the ram slots or reinstall them. I had a similar problem where the amount of ram shown was correct but Windows an Linux could only use half of the ram. It was a contact issue.