bring up MSI afterburner. increase the power limit to 50% - core voltage +50 - leave your memory at 1250 for now and see if your core stabalizes.
also go into Afterburner settings and check - enable hw control and monitoring/ enable low level hw access /voltage control / force constant voltage / disable ulps / under unofficial overclocking mode drop it down to Without Powerplay support. / check reset display mode on applying unofficial overclocking. you can check extend official overclocking limits if you want too but its not nessisary.
if you find something that works tell it to start with windows / minimized and in the tool itself under profile tell it apply overclocking at system startup.