An update, because I hate to leave threads hanging.
After much faffing, I have a few things to report back on.
- I purchased a certified displayport cable which definitely does not have pin 20 connected. In my case, this does not solve my issue.
- I also tried powering off the monitor, unplugging from the wall, doing the same with the PC and restarting everything over an hour later. Unlike many others I've found by googling, this does not solve my issue.
- I fixed the q-code 02 error and the error with the green boot LED remaining lit by installing Windows 10 release 1909. This allowed me to install the Chipset and GPU drivers. It also dramatically improved system stability as an earlier version of Windows I installed (I'm lazy and didn't make a new bootable stick - big mistake) wouldn't allow me to install the GPU drivers and kept locking up. Apparently Turing cards need newish versions of windows so I reinstalled with 1909.
HDMI continues to work without fail and when booting with it I get to q-code AA and all of the boot sequence LEDs turn off - happy days.
However, with Displayport, I still have the consistent problem that about one in every five boots results in code AA and no LEDs lit, but the other four out of five attempts result in no signal to the monitor and the white VGA light illuminating. However, with the graphics drivers installed and windows coming up, the OS seems to detect the fault and gives the card a kick, restoring the signal. The white VGA light remains lit, but the display comes up. What this does mean though is that I can't get into the bios via displayport unless I keep hitting reset until I get a 'good boot' and the white VGA light goes out.
I'm thinking of RMAing the GPU but would be interested in any further thoughts you guys might have.