Had that same motherboard it was pain in ***. Could go to 4.0 but would do the same thing meaning its useless OC. I don't suggest doing any OC on it, that motherboard simply cant handle anything.
On what you should be looking for are tmps of Package, socket and Motherboard in general, other things are speed drops and vcore drops. That motherboard dosnt have LLC meaning it will probably receive pounding on clock speed and vcore I think you already have that problem. Any drop larger than 100mhz is alarming and have to be reduced its clear indication that it isn't stable. In my situation LLC solved that little issue