At this point, I've had 6 Fermi's in my possession that I messed around overclocking with. 3 470's and 3 480's. Four of the six (and all the 470's) had this same issue ... the memory wouldn't OC for crap if you OC the core as well. So this issue is very common. The good news is, the core clocks matter a lot more, so ... OC the core as far as it goes, and don't worry too much about the memory. The Fermi cards have a good amount of memory bandwidth even at stock settings.