So, I took your advice Czar66 and had a nose at the Event Viewer... Interestingly "Kernal-Power / Event 41" !!!
After a quick google-around... how many times would someone bother to check your power supply, or least how the power is getting to your PC. I rearranged the power cabling at the back and gave the PSU its own dedicated socket (not via a surge protector / multi-socket)... and voila just had about 4hrs worth of crash-free aviation !
The stutters are still there unless I disable e-core #8 and if I dial back the display settings sliders (clutter, Scenery Details, Pre-load etc) to around 75-80% gives a nice sweet spot, and that's running the F-15E on one of the Caucasus' low level Mission / Training - an easy 150-180fps (obviously not VR)