Trying to find the cause of persistent memtest errors

[DKawalec]

Hello everyone,
I've been using my PC for over 3 years now with no problems; recently however I had several BSODs in a row (I'm using Windows7 Pro 64-bit), so I started investigating.
My hardware is as follows:
Asus P5Q3 motherboard
Intel Core2Quad Q9400
Geil GG34GB1333C9DC, 2 sticks per 2GB, it is on Asus' QVL for this board.
Radeon HD4870 graphics card

I reinstalled some drivers, chkdsk was OK, so I run memtest overnight and in the morning there were over 9 thousand errors after 9 passes, so a thousand per pass approximately. I started testing each module individually in different slots, and each of them passes the tests with no errors, but as soon as I plug them both in (into proper slots in accordance with the MB manual) memtest errors are back, but now there's between 3 and 10 of them, during the 5th or 7th test, and always at the same addresses. Windows Memory Diagnostic passes with no errors, but memtest brings some up every time.
Having read that it might be a BIOS issue, I tried updating it, but it did not change a thing - also I had issues with the update. I went from version 0804 to 0806, it resulted with a blank screen on reboot, so I unplugged the PSU, pulled the battery from the MB, waited for about 2 minutes, then replugged everything and the computer booted normally, only now it reports 0806 version installed - so I guess the update was successful after all..?
All memory-related settings in BIOS are set to auto, I never overclocked it or changed anything, as I don't know much about those things anyway.

My question is - what could be causing this? Could the memory sticks be faulty, yet still pass the individual tests, or is the issue in the motherboard/CPU? What other diagnostics could I perform that would help find the culprit?

Any help would be greatly appreciated, thanks in advance.

 

[swilczak]

Yes one or both of the memory modules could be faulty and still pass the memory test, I have had this happen before. A faulty memory module is a very common problem. In the past few months I have fixed several PC's with bad modules. You can take one of the modules out then run the computer for a while, if the problem goes away, then the module you took out is bad, if not put it back in and take the other one out and run it for a while. You could go to the properties of your hard drive if this doesn't work and run an error scan which will take a while.

 

[DKawalec]

Thank you for your suggestions!
All the hdd scans reported OK; the BSODs stopped appearing for some reason, no matter how many memory modules I inserted, but the memtest errors remained.
So I reported the modules to the producer as likely faulty and sent them over to be checked based on their lifetime warranty; gonna wait and see how that goes.
I got a replacement set (Geil Black Dragon CL9, 2x2GB) since I can't afford to lose access to my PC now, and lo and behold, one of the new modules is detected as faulty by memtest right away! I'm starting to think Geil was not such a good idea in the first place.