Can't do stock timing

I bought two sets of these 6gb modules:

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

for a total of 12gb. The spec timings are 8-8-8-24 at 1.65 (2000MHz)

I OC'ed my machine to 3.8GHz with ~1400MHz dram frequency. The mobo is on auto memory timing and uses 9-9-9-24. Everything is stable at that rate. When I try to manually set 8-8-8-24 and 1.65v, my memory becomes unstable and fails memtest. Shouldn't this memory be able to perform at the specified timings? Or is there another factor I forgot about?