Card or Power Limit?

[Cali3350]

Hey guys, I have a GTX 260 that right now maxes out at 702/1404. Im fine with this overclock, however what bothers me is what happens when i try to increase this amount. I put the card at 729/1458 and it runs fine for an extended period of time however inevitably locks up the system after enough time. No graphical errors or anything, just locks up. Does this translate to a limit on the card, or is it a voltage issue or something causing this? Any ideas welcome guys, thanks :)

[Bman212121]

It's most likely that the GPU on the card needs more voltage to run at those speeds. At 700mhz it's already quite a ways above stock