It's always difficult when you have a problem with RAM knowing exactly what is best. If you bought the original RAM modules as a kit and one stick becomes faulty it is always better to RMA all of the sticks not just one of them. Whilst in the vast majority of cases you can get away with mixing and matching RAM in reality it is not the best way.
The fact that you have now run an extended memory test and errors have come up would suggest to me that you have been unlucky with your RAM. Although the memory controller on the CPU doesn't like running RAM above 1333MHz, the truth is it is pretty uncommon for it to fail.
The options you now face though are:
1: Replacing the RAM (again) this is the option I would go for as it is more likely to be faulty RAM than the CPU.
2: Contacting AMD and seek an RMA on the processor. If you chose to RMA the CPU, it will be tested by AMD and returned if it is not faulty but if there is a problem it will be replaced. Either way you will know that your CPU is OK.
As I said right at the beginning, these kind of problems are often the most frustrating and difficult to cure because there is a 50/50 chance that you will replace one thing only to find that it is something else causing the issue.