Agreed. That's how I found the problem in one of my Corsair memory modules. Also agreed about trying the other slot. Kill two birds with one stone and try the other DIMM in the other slot. If you still get a failure then we can all agree you've ruled out RAM.

Sorry to keep harping on the RAM despite Memtest telling us you're fine, but everything you're describing matches my experience with an intermittently bad RAM stick.