Computer randomly drops video output from new GTX 970 and old GTX 570

5-6 months ago I had a series of random video loss events where the monitor would suddenly go dark, then display the "DVI no signal" message and go to sleep. At the same time, the speakers played the "USB device disconnect" two-tone signal- something like "BEE-doop" with the first tone being higher and the second tone being lower. I tested the monitor and its cable on another machine, and they worked fine.

After weeks of fighting with this, I finally gave up and decided to replace the "most likely candidates" without ever knowing what the actual problem was. I replaced the GTX570 with a new GTX970, and the old Gigabyte Z77-D3H mobo with a new ASRock Z77 Extreme 4. I then fresh-installed my Windows 7 Home Premium x64 on a 64GB SSD and all my games onto a 264GB SSD, retaining my old 2TB HDD for Documents, downloads, email etc. that don't benefit from SSD speeds. The only remaining "old" components are the monitor and its DVI cable, the CPU (Core i5 3570K), the RAM (Corsair 8GBx2), the PSU, and the case, CPU cooler and fans. This completely solved the problem, and for the past 5 months the system has been rock-solid and stable, even through marathon 12-hour gaming sessions. The exhaust air from my fans is barely warm, and my CPU stays around 35C no matter how hard I'm pushing it. I am not overclocking either the CPU nor the video card.

Suddenly last week, I got the dreaded "BEE-doop" disconnect sound out of a clear blue sky, the monitor went black and displayed the "DVI No Signal" message and went to sleep. Meanwhile, however, the game that I had been in the middle of playing (heavily-modded Skyrim) was still running! I could hear the NPCs talking to "me" and the Bard still singing in the Inn we were in. So obviously, the computer was still chugging right along even though the monitor thought it had died. Without any display, of course, there is no way to "nicely" shut down the computer or even reboot it, so I have to press the power switch once, which shuts down the machine, but scatters and rearranges, and even loses some of, my desktop shortcuts and icons.

Since that time, the same thing has occurred at least once a day, and last night when it happened, I got not one but about 5-6 "BEE-doop" disconnect sounds in rapid succession. Very odd. The second or third time it happened, I disconnected and reconnected all of my cables, removed and reinstalled my RAM sticks and GDU, wiggled all of the power connectors in the machine and vacuumed out the little bit of dust that was on the CPU cooler radiator, but it kept happening anyway, about once a day or so.

It just happened again today, so after I rebooted I checked Windows Event Viewer, which shows zero "critical" events and 3 warnings when I look at just the last hour, which more than covers the most recent crash:

219 | Kernel PnP | System
3057 | Application Virtualization Client | Application
3191 | Application Virtualization Client | Application

Of course, I have no idea what those warnings mean, but since they happened during the past hour, the time of the crash, I suspect they may be related. Also, "Kernel PnP" seems to go along with the "device disconnect" sound I'm getting at the instant of the crash. No other events of any kind have been logged during the previous hour.

I am now in the process of pulling a complete backup off of the machine onto an external drive, because when this happened 6 months ago it ended up corrupting my HDD and I was eventually unable to boot the machine at all, even using the on-board display.

Meanwhile, I am at a loss how to troubleshoot or diagnose this recurring failure. Logic would suggest that since this issue is identical to the one I had 6 months ago before most of the hardware was replaced, it must be something that was NOT replaced causing the problem- but then again, logic also suggests that what I just said is invalid since it ran great for 5 months after I replaced most of the hardware, indicating that the components I did NOT replace were probably OK or it could not have run so well for so long.

As you can see, my head is spinning because none of this makes any sense to me and my "logic" is leading me to two contradictory conclusions. Does anyone have any suggestions as to how I might isolate and fix this problem?