So I pulled the Tesla M2090 card from my system and checked the heatsink for blockage. No issue there so removed the heatsink from the card;
Carefully removed the old silver heatsink paste (which was starting to go hard) using a wipe and some methylated spirits then re-applied fresh silver compound heatsink paste before re-attaching the heatsink. Re-fitted the card to the PC and switched on. Booted to windows as normal and everything was looking okay. Tried to re-run octanebench render benchmark program but the computer went to the black screen again after about 2-3minutes into the test. The fact that I was able to run the benchmark on a 'cold' PC would indicate the PSU can cope with the power demands of both GPU cards but perhaps the M2090's passive cooling may not be upto the job. When you see how tightly the cards are fitted you can see air flow may be a major issue;
I will look at ways to improve the cooling this week.
Just some more info on boot times for my PC. The times are based from the moment the power button is pressed;
1. Windows Logo appears after 35seconds
2. Logon prompt appears after 1min 12seconds
3. Windows fully loaded (with a ton of background apps and VPN) after 3mins 40seconds