As someone who has owned 2 MSI Gaming 970 mobo's, I can tell you you are pushing that board to its very limits. It has cheaper 6 phase VRM's that don't handle a lot of power. I can post a pic of a burned VRM on one of mine to illustrate that fact.
That 480 being a 6 pin connector is drawing part of its power through the pci-e slot, putting more stress on the VRM.
480's aren't strong overclockers. It's just a fact. Having stable, clean power is a must to get any real clocks. And even then, you may not win the silicone lottery.