Matched RAM checks good individually, fails if combined. Is the RAM or mobo the culprit, and what next?

I'm troubleshooting a problem with my desktop PC, which has an aged but still sufficiently decent Asus Sabertooth X58 motherboard with three 4GB sticks of Mushkin 9-9-9-24 DDR3-1333 RAM in it. I'm getting some really weird behavior, as follows:

• With all three sticks installed, the BIOS set to 9-9-9-24 and either 1333MHz or 1066MHz modes (more on that in a minute), Memtest86+ set to multi-core mode reports intermittent (but fairly frequent and very lengthy bursts of) memory errors, but doesn't tell me which stick or slot they're being detected on.

The reason I've tried both 1333MHz and 1066MHz is that my BIOS mentions that it only allows 1066MHz or 800Mhz options unless your CPU is unlocked. My CPU is a Core i7 950 which, according to much Googling, is probably locked. But the RAM itself, which has worked fine for years, is marked as being 1333MHz. I've tried both 1066MHz and 1333MHz options in the BIOS for all 12GB (failed for both speeds), and I've done all of my testing the individual cards at 1066MHz since that's what the BIOS recommends. It fails at the lower clock speed only if more than one card is installed.

• All three sticks have been successfully tested individually in the PC through one full pass (~90 minutes) apiece in all three slots. (Well, the final stick is in the final slot and a third of the way through its test as I type this, but I'm pretty confident it's going to pass as it's already done so twice in the other slots.)

So that's three passes per card, and three cards tested per slot, meaning every component has been individually tested good through at least three full passes (or 4.5 hours of continuous, intensive testing).

Some cards / slots have actually been tested more, as I forgot at one point which was my tested / untested pile so had to do one slot all over from scratch. Plus I did a fair bit of testing without errors before I found how badly it was erroring in multi-core mode with multiple cards.

• Here's the weirdest thing. All three cards were sold to me as a matched set in a single blister pack (actually, replaced as a matched set a few years back when I had a single card from my initial order fail), and despite all being identical in every way, I can only get the motherboard to detect 4GB of the 12GB total unless I put specific cards in specific slots.

(And I mean slots on the same bank here. I'm using slots A1, B1 and C1 of the six total provided by my motherboard, and those are the ones the manual tells me to populate first.)

Given that I've already tested everything individually, it seems to me like either it's an interaction between multiple cards, or some kind of timing / other motherboard issue which isn't sufficiently stressed until more than one stick is installed. Or it's a bug in Memtest86+, but that seems unlikely to me given that it's a very well-known and broadly-used, long-established program.

So...

Do I now need to go through testing combinations of two cards at a time and two slots at a time to try and see if there are errors at this point too, and if so whether they suggest the memory cards or slots / motherboard to be faulty?

Or have I already established that the fault must lie in the motherboard itself, given that every single card and slot has individually checked out good several times. (Which it's now just 45 minutes or so from completing on that final card.)

Also in case it's relevant, the original problem I'm working on was sudden, major corruption that (while Windows itself started) prevented many programs -- even basic ones like Explorer -- from working. The boot drive has tested out fine when hooked up to my laptop, and I've been able to extract all the data from it and wipe it. Nor does it trip a SMART warning. So the drive itself seems to be fine.

I'm really suspecting the motherboard here, but I don't know if there's something I've missed. I'm kind of at a loss for where to go next, and money is extremely tight so I can't afford to replace both motherboard and memory unless I absolutely need to. (Nor do I really want to research what will work with all my other components properly and with good performance / value these days, as that's a big task in itself.)

Any advice most gratefully accepted!

Submitted February 01, 2018 at 05:10AM by gweilo8888 http://ift.tt/2EsMoY3