Boot (or wakeup) normal for one sec ... then black screen

My wife's desktop is giving an intermittent problem. She usually sleeps it overnight, and I wake it up in the morning.

Beginning a few days ago, the wakeup would arrive at the logged-in Desktop for about one second, and then the screen would go black. A head scratcher. All I could do was a cold-shutdown.

On restart, the usual HP logo blue screen appears (with "press Esc to get boot menu", etc.) Then the normal glowing Windows flag during boot. Then the user login screen for a second ... and then black.

Re-reboot! This time I press Esc to get the boot menu. This menu is a character graphic showing three choices: DVD device, Realtek device, Hitachi hard drive. I down-arrow to the third choice (hard drive), press Enter, and the boot completes normally. I can login. No more black screen.

Next day, same thing. (She shutdown this time, so I wasn't waking, I was booting). But this time the three-item boot menu appears for a second, and then black. I could still down-arrow "in the blind" to the third item on the list, and press Enter to boot ... and the system behaved normally.

Next day ... resumed from sleep with no problems! (And that was on April Fool's Day) :)

But no, this is not a joke! All of this really happened.

So whuzzup with this box?

Could it be a temperamental monitor?
Or a wack cable? (This monitor is on a swing arm, and it gets pushed around multiple times every day. The cable has been flexed around that way for years. I'm tempted to shove a new video cable in there.)
Or could it be some Windows gremlin?
Or could it be that we removed AVG and installed Avira a few days before this started happening? (Seems far fetched.)

Dunno.

I used AVG's uninstall tool to remove AVG.

The problem appeared again this morning. It was sleeping, and upon wake up the desktop went black.

I removed and replugged the DVI cable from the computer. Flash on, flash off/black. No good.

Then I tried turning off the monitor and turning it on again. Success!

I'm still not sure whether the problem lies with the cable or with the monitor. But I have lots of spare cables, and two spare monitors in storage in the attic. So it's time to do some swapping.

I swapped the DVI cable. So far no problems.

So ... new DVI cable last night ...

My wife shut it down late last night. I booted this morning early. Normal.
A bit later I put it to sleep and then awoke it. Normal.
I put it to sleep again and went to work.
My wife called late in the morning ... black screen. Back to square one.

It turns out that we have three identical monitors. Two have been in storage for several years, unused. So I swapped one in. So far, so good.

your troubleshooting steps so far have been the right ones to take imo. It would not hurt to also have a bootable non-windows environment to run, such as hiren's boot cd, UBCD, bootable linux, anything. Just to rule windows out as the cause. Or not.