Well found out it wasn't the vertical mount bracket and gen 4 riser, It was the CableMod direct PSU 12VHPWR cable that was the issue, Both the screen would go black and fans would spin up 100% if any load was put on the GPU, Noticed others commenting on roughly the same situation when the 3090Ti came out on random reddit threads all of whom also bought custom CableMod cables so thought I'd try to switch back, Put the stock Nvidia octopus cable back on and it's again 100% rock solid stable, Ugly but stable.
Now the tedious task of RMA'ing a faulty £50 cable back to China....
Also noticed something quite odd, With the CableMod cable, And this is fully repeatable, In MSI Afterburner I cannot select anything higher than 100% Power, With the stock Nvidia cable it goes up to 111%, So CableMods cables are doing something funky.