Unscrew the rad part of the AIO and turn it the other way around so the tubes are at the bottom then retest. If it is not totally full then the pump could be struggling to push the water around the aio.
Grim5 gave great advice. If the temps rise extremely quickly then it is a mounting or pump issue and not airflow/radiator issue.
Agree and will try a different cooler should this not work.
I can't afford to have a £700 cpu getting to those temperatures that quickly and need to rule cooler out or as culprit before looking to RMA cpu.