Memtest86 is a great and invaluable
tool, but by no means a "be all, end all" to memory stability. I find it invaluable for "scouting out" timings and settings for optimal stability, especially with it integrated into the motherboard's BIOS. 3D stability is a common scourge of an otherwise clean overclock, and always needs to be taken into account, memtest86 aside.
I use memtest86 for stability testing, and I use Prime95 for stability testing,
many people don't like or trust these two programs - I personally firmly stand by them.
But there's no way on earth I would trust them as an indication of proper and thorough stability by themselves!
True stability involves an overclock passing memory specific, processor specific, and GPU specific/blended stability testing. For me, that means Prime95, memtest86 tests #5/#8, and the 3DMark2001 Nature test looped.
I can generate overclocks on my current machine that will pass any memtest86 test for ~20 hours, but fail Prime95 or SPi. I can generate overclocks that will pass Prime95, but lock the 3DMark2001 Nature test. I cannot generate an overclock that will run unstable after passing all three
My two cents.