STM32G473 will not work at 170MHz but works fine at 150MHz

I have a very odd problem. I am using the STM32G473 and have configured the clock to 170MHz. When the code runs it drops to default_handler every time after setting up the clock. Not unusual if there is no ISR or call back set up. However if i set the clock to 150MHz it works perfectly. To run at 170MHz the R1MODE needs to be cleared in the PWR register i have done this. Is there another register that needs to be set up to operate at the full 170MHz.