Memtest results....confusing

Ive been running memtest on my RAM lately, when I run it with 2x 512 sticks in, after a few hours, I start getting errors at certain memory adresses (I dont know the specific time, as I cant watch it for 7 hours). So I took one stick out, let it run for three hours....no errors.....I thought that one might be ok. So I took that one out and placed the other stick in the same module I removed it from in the first place. This one ran for 10 hours, with no errors.

So....3 hours without errors on stick 1, 10 hours without errors on stick 2. Yet I get errors after 7 or less hours with both sticks. Could this mean that stick 1 is the faulty one?

Or is it possible that, even though the two RAM sticks are exactly the same, and have no errors on their own, combining them creates new errors? Confusing as that may seem.

Help would be appreciated here, thanks!

PS: Its DDR RAM, not DDR2 (Im thinking of upgrading soon anyway, cos theres a DDR2 motherboard round here somewhere)

Be explain the need to perform a memory test in the first place? Crashing PC, slow? Since Ram is so cheap I would just buy whole new Ram.