The 600i and 700i chipsets have issues that are directly related to memory and voltage. 600i's were far worse in both departments. These chipsets are probably the only ones in damn existance where you have to up the NB voltage to get a decent oc....its absurd. This does go for EVERY board though, they are extremely inconsistant.
I have the evga 790i sli ultra and its been on and off again with problems. this is with either no o/c or a slight o/c to match fsb to memory. the most difficult thing i found with this board is the memory timings and voltage. it does not read it correctly or just does not detect the correct voltage the ram is supposed to use. default voltage for the mb seems to be 1.5v but my ram requires 1.9v i have to manually set this every time my bios freeks out which seems to be about once a month. there are also other stability issues i have not isolated yet.