Doh, sorry, don't know how I missed that in your first post. Did you try each slot or just the 1st slot? Also, if you are overclocking, try resetting your board to defaults.
Also, like I said before, failing memtest is enough of a reason that G'Skill will exchange the memory for free. I've had to do this a couple times in the past with some of their DDR2 dual channel kits. They are very good about lifetime warranty policy. This would at least eliminate the memory as a problem.