Do you have C1e Enabled in your Bios? I don't know what board you have but on mine C1E drops the multiplier when idle as a power saving device. diasble it and it will stick at your settings.
If you plan to overclock leave it off when your trying to get your overclock stable so you can rule that out as a factor. When you have a stable result then you can switch it back on to save power BUT, run your stress test one more time to be sure it won't suddenly make it unstable again. And then you've found the end of the rainbow.