OK, based on you testing scenarios, i'm guessing it's a motherboard's issue, BUT nonetheless, mixing RAM modules could be also the problem.
Before all the test with memtest86+, do the following:
1) test DIMMs 1/2 with one set of RAM (the 8-8-8-24 ones), let the bios automatically adjust the timings and voltages
2) same as above but adjust the timings yourself on the BIOS to the RAM specs
3) test DIMMs 3/4 with the same RAM set (8-8-8-24) and do both, automatically adjust and adjust timings on BIOS
Then, do the same 3 tests, but with the other RAM set (9-9-9-24) write down every single result and then post back with them
That way we could determine if it's an specific RAM set or if it has something to do with the fact that you mixed different RAMs.