ASUS P5N32-SLI SE Deluxe troubles

[Slavedriver]

I had two ValueSelect 512Mb 667Mhz (VS512MB667D2) modules and recently bought 1Gb kit (2x512Mb) ValueSelect to add myself another Gb of RAM. The problem starts when I insert all 4 planks of memory. Windows refuses to boot (says some system file is missing and the file is not always the same) and when I try to reinstall it I get BSODs at different stages of install one way or another connected to the memory. When there are only 2 planks inserted (new or old) memory works on 553Mhz frequency. When all 4 are inserted it is suddenly 667Mhz (like originally was intended). I haven't done any overclocking, voltage adjustments and so on. Memory seems OK (Memtest hasn't shown any problems).

Again it happens only if all memory slots are filled. It works fine with 1, 2 or 3 planks placed in random slots.

 

EDIT: Forgot to say. BIOS is the latest available.

EDIT: Tried upping voltage to 1.9V. Didn't help. Also I occasionally can't even POST and have had to reset CMOS a few times already to even get into BIOS.

[RAM GUY]

Run http://www.meemtest.org on the modules one at a time to insure one is not failing.

[Slavedriver]

I previously memtested them all 4 at once and 2 new and they were OK. Now I listened to your advice and memtested them one at a time and they were OK too. Then I went and downclocked memory to 533Mhz and Windows booted fine. But as far as I can tell this mobo does support 667Mhz RAM... What else could be done about the problem ? I also dunno about stability because I just booted up happy that I got it at least somehow working.

[Slavedriver]

Umm...bump ?