There's nothing you can do JJ - tbh I'm amazed speeding it UP made it more stable but there you go.
As for the other brief isue, I suspect the good memory probably didn't boot cos you either didn't install it correctly the first time, or the timings were messed up.
Anyway you got 2 options
1) Go to the website of whoever MAKES the memory (ie.
www.crucial.com or whatever) and RMA it if it's still under warranty period - they won't know (and won't care) that it came as a replacement from scan, it's just dodgy ram so far as they know.
2) Roll with the afformentioned chuck in dustbin/give to someone you don't like/chuck in the dustbin of someone you don't like kinda suggestion.