Question : Problem: Memtests

I have a computer here - I am running the Memtest 86 V 3.2 bootable cd. I cannot figure if the problem is the MB, or the RAM. I have 2X 1GB chips of PC2-6400 (DDR2-800mhz), and 2X 512mb chips of PC2-6400 (DDR2-800mhz) and four slots on a capable motherboard. The mother board model is P5N32 -SLI SE Deluxe. When I put in just the 1GB chips in either set of ram slots it tests fine - (2X 1GB).  It tests fine with the set of 512mb chips in any set of slots as well. But I have the following questions:  Why will I sometimes get 1 or 2 memory errors one test and other tests it tests just fine? I've never had that before, and I am sure the RAM is fine because I've tested it over and over. It is all the same kind of ram. Could the motherboard be a problem?  Also, the MAIN question is - why does it take 6 times longer to test the 2X1GB chips alone in the system than it does to test the 2X512MB chips?

Answer : Problem: Memtests

that is so for every mobo - you can also use the ram selector from the ram's vendor site