@holger1405 When it happens it just shuts down completely; no blue screen, and the monitors go black too. So seems like an overheating issue still, but it's strange when I'm keeping everything relatively cool.
Only run a Memtest and used Heaven Benchmark but both had no issues, it's still just something about Apex.
Haven't tried 3DMark, Time Spy or Firestrike yet, should I try them all? Will start with 3DMark and let you know.
My next step though is to try an OS reinstall at some stage, and if it still happens I'll look into a more powerful PSU because the RTX 3090 does seem pretty power hungry.