This is how it went:
One of our draftsmen informed some flickering video issue which I tried to replicate unsuccessfully. This is a WIN7 64, Gigabyte H77M D3H MoBo.
On one of the restarts, all looked just fine, windows logon sound included but no video. Everything was double checked all good but video. No splash screen, nothing but the natural flickering when the monitor goes from stby to active or from hibernation to stby. there was no way of knowing if it had problems. Even the digital/analog prompt showed and all. Then, the screen just went back to stby and hibernation.
Next, the GPU (GTX550) was tested on a second unit: WIN7 64, Asus Maximus IV Gene Z Mobo. All different but the Monitor which was still connected to the electrical plug all the time. Same results. Windows logged ok, no video at all. The GPU seemed to be the issue.
Checked forums, followed standard procedure on the Asus: flashed bios, stripped MoBo, connected one thing at a time, etc. got A2 (IDE Detect) code on the MoBo debug panel. No video.
Next, the Asus Unit is re-assembled as it was before the problems, this time with its usual GPU (GTX660). Nothing.
When I checked forums, found out that these symptoms or apparently the same problem (A2 code + no Video) have been given RMA processing a couple of times. So I prepared to fit the drives on other units to preserve data and keep on working.
Now I know both MoBos were stuck on BIOS settings, since they've got BIOS' flashed. All the time same monitor, plugged to power. But there was no way of knowing what was going on for not having video. The Asus was expecting F1 to fix CPU fan low rpm because it has an Antec Kuhler. The Gigabyte H77M was also waiting for BIOS settings due to IDE config (AHCI) for the SSD drive.
Connected the H77M to a TV via HDMI and it worked, so there was no problem with the MoBo but then, what about the GPU's DVI sockets?
Tested both on another monitor via DVI and all was ok.
So, I thought the monitor was damaged. Finally, pulled the monitor's electrical plug.
Next day, tried to check the monitor, all ok.
That's pretty much it. The monitor just needed to be reset.
I believe it happened because the DVI plug was loose from the socket and this got the Monitor stuck. It's a Samsung SyncMaster that works flawlessly.