Mounted a STM32F405RG from a new batch I have and it works perfectly at 168Mhz. It just hit me that the other board have a ADM2582E mounted so I removed that and that board/MCU suddenly works at 168Mhz as well. It’s no question, I moved the ADM2582E to the New board and the board suddenly don’t work at 168Mhz.
ADM2582E contains a DC/DC converter that seems to be the cause of problems here. Reading the datasheet and app notes I seem to have stepped into a few issues on the design/PCB needed to reduce EMI noise – it’s a 180Mhz oscillator frequency for DC/DC used on that chip …