So, just to recap...
You're not overclocking at all?
You've tried both sticks in slots 1 and 3, and that gives errors.
You've tried a single stick in slot 1 and that gives no errors.
You've tried the other single stick in slot 1 and that gives no errors.
You've also tried a single stick in the other slot used for dual channel operation, ie slot 3, and that gave no errors?
You really need to try a single stick in all four slots in turn, to see if it only happens in one of the slots.
Next, try running both sticks in slots 1 and 2, and then 3 and 4, which is single channel mode, and see if you get any errors on those.
I presume you had your ram in slots 1 and 3 or 2 and 4 when you first got the errors?
It sounds like you're ram doesn't like running in dual channel mode?
I've had similar symptoms on one of my setups, and that turned out to be the motherboard after trying all of the above configurations had ruled out the ram...