I put these 2x4GB of F3-14900CL8D-8GBXM on a Maximus Formula III with an I5 750, and both with everything at default, then setting it manually 1866 (with timings and voltages fixed by specific or also to auto) when launch memtest86 + (also memtest) from UBCD (tried both last version that the previous one) that crashes immediately with a continuous beep. Yet in windows I tested the stability with 10 cycles linx to set up and does not give me errors. According to you, it might be some sort of incompatibility of these memories with memtest tool or with the motherboard, or is there really something wrong?
I tested in windows with 1hr of linx and 3hr of prime and its ok, with all at default setting, but memtest86+ not work at all (I thing is an incombatibility of this ram with a old motherboard like mine and memtest)
Try bumping your ram voltage to 1.55 or 1.6 and see if it will stabilize Memtest86+. Do you have the latest bios on the MFIII ?
