Do yourself a huge favor and purchase a brand new set of matching RAM sticks, and install.
Problem solved.
To do that:
1. I would remove one of the 2 and let the test run overnight.
2. Put back the first and remove the other and let the test run overnight.
Correct?
I think so, that way you'll find out if one of the sticks is defective or if the sticks just don't match each other. You'll probably need to purchase 1 stick anyway, so instead you may wanna follow TorrentG's suggestion to purchase a brand new set of matching RAM sticks. Or get a replacement.