The issue is that the PC seems to run absolutely fine for around 30-45 minutes but then shuts off. It doesn't appear to be a CPU issue since the temperatures are fine throughout the use of the machine (idles around 30 degrees Celsius), I am also able to turn the PC back on straight away without any issue except sometimes I need to remove the front power connector and place it back in and then it boots up fine.
I'm unsure as to what the root cause of the issue is but any help would be much appreciated, thanks.
since you write that cpu stay around 30 at idle what is the temp on load for the cpu and gpu also check if your fan are spinning on both of them .
Tested the machine with Prime95 and left running for about 20mins yesterday (manually closed it after that time), max temp reached by the CPU was around 70 degrees Celsius in that time. As for the GPU it idles around 40 degrees Celsius and on load goes up to high 50's / low 60's and yes the fans are running on both the CPU and GPU at around 1300-1400 RPM.