That's a RAM failure for sure. Some RAM failures are tricky to detect, if you look at how RAM actually works at the electron level it's a wonder it works at all! The general opinion is that 8 runs of Memtest are needed to detect about 98% of potential RAM problems, the key is to stress each bit of RAM hard enough and long enough to detect a problem. I asked you to do two runs because the free version of Memtest only does 4 iterations, and for hard to detect RAM issues that's not really enough.
You could try reseating the RAM and see whether that helps, sometimes it does.
You could also try testing each RAM stick to find the one that's flaky and then just replace that - with exactly the same make and model of RAM. If you can't find exactly the same RAM then make sure that all voltages and timings on the new RAM are the same as on the existing RAM. However, you really want your RAM in matched sets, and ideally from the same slice of silicon, because they need to operate as one. For best reliability (and performance really) you're better off replacing all four RAM sticks with a single pack of four matched RAM sticks. It's up to you whether you want to go to that expense though.
