No video output from GPU at first but then works later?

Specs:

-Intel i5 3570k @ 3.4ghz (not overclocked), Noctua cooler.
-Asrock z77 Extreme 6, BIOS version P2.70 (LATEST as of 20th July 2013)
-ASUS GeForce GTX 670 DirectCU II OC 2GB
-G.Skill F3-14900CL9Q-16GBXL 16GB (4x4GB) DDR3
-WD black 1TB HDD 7200.rpm (Purchased 2011)
-Antec True Power 750W PSU (Purchased 2009)

Setup as above, turn on PC with DVI-D cable plugged into GPU and the screen displays nothing. Use an old VGA cable and plug it into the screen and then into the motherboard and Windows has loaded and is at the log in screen by this stage in a very low resolution ( My screens' native resolution is:1920x1080 but the resolution displayed is much lower).
I restart the PC (with the VGA cable plugged in) and open BIOS and in the System Browser the interactive display says there is nothing in the PCIE2 slot where I have placed the GPU.
I then exit the BIOS and log into windows and leave the PC for 20 to 30 minutes. Come back and shut down the PC entirely then turn it on again this time with the DVI-D cable plugged into the GPU and no VGA cable and the screen is still blank unless I hit F2 to go into the BIOS at which point the screen registers video output from the GPU and displays the BIOS.
I can then exit the BIOS and log into windows and everything works as it should. However when I shut down the PC and leave it for more than 5 hours and come back to turn it on I have the same problem and have to go about the above method to get the PC to work properly.
I have a 750W PSU which is 4 years old that I have tested with a PSU tester and the BIOS displays the power output to be within range. My hard disk drive has been tested by Data Lifeguard Diagnostic for Windows which says my HDD is fine. I have slotted my GPU into another PC and it has worked fine.
I've been trying to figure this problem out for several months now and I'm not sure if it's the motherboards PCIE2 slot that is randomly functioning and then not functioning or if it is a software issue. I have also tried plugging the GPU into another PCIE slot and I still have the same problem. I have even 'ghetto'ed' a piece of plastic to prop up the GPU so the cooler doesn't pull it down at one end but that doesn't seem to be the problem either.
Yes, I have the cables plugged in properly and the GPU and all other components are all powered properly as the PC will work after waiting 20 minutes an the turning off and on and entering the BIOS. I just don't understand why I have to start the PC this way. I have also done several reinstalls of windows to no avail.

Summary: plugging my screen into GPU with DVI-D doesn't work at first but windows is loading and I can see this with a VGA cable plugged into the mobo, after leaving windows logged in and doing nothing for 20 minutes I then shut down my PC and turn it on again and with the DVI-D cable plugged into the GPU I hit F2 to go into the BIOS which switches the screen to the GPU output.

Any help would be much appreciated as I have already sent the mobo and GPU back to the store and the mobo was then sent back to Taiwan to be tested and they both came back after 3 months (.......). I have been banging my head against the wall for almost 4 months now as to what this problem is and any help would be much appreciated. Thank you for your time.

Rooster.