Hi and welcome to TSF.
A 0A stop error can be caused by several things including a hardware change. As the problem occurs when both sticks and in and occasionally when the old stick is in alone, it does look as though this could be the problem. the first thing to do is run a test on the original stick. For this you can use
Memtest86. You will need to download the ISO file and burn Memtest to a CD and boot from the CD. The test takes a long time to run. I usually leave it to run overnight. See what results you get with that and post back.