I would suggest to use Goldmemory to test memory, I've done endless testing in last few months, and goldmemory catch memory error simply better, always 3-4MHz before Memtest and in line or even before that Prime Blend. It's not free but a shareware version with some limitation is available to download.
The difference about what Memtest report and RAM stability in OS is all about the way that it's stressed, Memtest can't reproduce the access pattern that a OS multitasking do.
Different memory testing software use different algorith so is usefull run them all for extra confidence.
Goldmemory is the best to do the job, I've never seen memory stable for goldmemory but instable for Memtest, Windows Memory Diagnostic( a nice program by M$ but never updated to final release), DocMemory.