Depending on which Asus motherboard you have they can run unstable at lower latencies.
My Asus A7N8X-E-Deluxe only lets my memory run at 2.5-3-3-6 if the frequency is 195MHz. Hopefully Asus will release a new BIOS that corrects this issue.
nForce2 chipsets are renowned for being tight of CAS latencies.