So, it's a Custom-build desktop then. What about your Power-Supply spec? What Wattage and Brand is it; flickering if not caused by a defective GPU or external Video Card is often about having insufficient or unclean Power to the +12V and +5V rails.
I'd be willing to bet it's the hard drive or your PSUgiven that you have a custom-built machine. Most folks don't spend enough money on their PSU. Is you PSU a 80+ Certified? If not, that could be one of your problems.
Have you attempted your tests with a known working Monitor that works on another comparable custom-built with a similar or identical video card to the one you have? Running the same video or game??
In order I would suggest you do this:
1) Run the SEATOOLS drive diagnostic to test your hard drive
2) Post back EXACT specs and power on your PSU you are using; and we can tell you if it's adequate or not
3) Perform the monitor Swap, if you haven't already done so
4) If your ASUS Motherboard has an on-board or embedded Video chip; remove your Zotax video card from the ACP or PCI16x slot and change your BIOS over to boot from Internal or Auto to boot from on-board GPU chip. Retest with Video. If it works; then it's your Video Card that's failed. Return to place of purchase for exchange or replace if out of warranty.
5) Reinstall Windows8.1 on your existing hard drive if it passed BOTH tests on SEATOOLS; if you replaced the hard drive because it failed SEATOOLS, reinstall Windows8.1 on the replacement drive
6) If #1-#5 fail to solve problem, it's most likely a PSU problem which you will have to replace, or a faulty Motherboard, which you would also have to replace.
Given an adequate PSU, tested hard drive, RAM, and a complete Windows8.1 reinstall from Factory Media; your Motherboard would be the most likely component that's failed and would then need to be replaced.
Good luck,
<<<BBJ>>>