I can almost guarentee by your comment
Each would work seperately once, but one wouldn't work if you rebooted
You have a dead stick of RAM... or a stick going bad.
The stick you have in right now is the good one but run Memtest on it anyway, do it before you go to bed and let it run all night long, that way you will be sure. As for the other stick, if you can get your PC to boot up then run memtest on that one as well, if your PC won't boot with that stick, or constantly freezes... well, there's your problem, the RAM is bad.