Screen Goes Black Frequently

[heavydude]

This started suddenly while I was watching an old movie on youtube. The screen went black for an instant and came right back. Had no effect on what I was watching. Happened again a minute or so later. Then continued to happen at shorter and shorter intervals until it was seconds apart, still not effecting anything that the computer was doing. Just got so annoying that I had to shut it down. Now, when I turn on the computer and just leave the desktop on without bringing up any websites or doing anything else, it starts doing that about two minutes after the computer finishes starting up.
History: The computer is a 7 year old Compaq Presario desktop with Windows Vista Home Premium 32 bit with Service Pack 1.
In July 2012 I got another computer for my office and set the Compaq up in the den and hooked it to a TV by HDMI so that I could watch TV shows and movies with the computer.
To do that I had to upgrade the video card and got an EVGA GeForce 210 1G. With that I upgraded the power supply to 460W.
In April 2013 the video card went bad and EVGA put me through 9 yards of you-know-what and then sent me another one.
Subject to comments by experts, I suspect it's the video card again, but it might be something else.
Comments welcome.

 

[TrainableMan]

Choice is the video card, bad or loose cable, or the monitor/TV. A different cable and TV are probably easiest way to try and rule those out.

 

[heavydude]

Tried different cable, no help. TV is relatively new and works fine independently of the computer. I pulled the video card out and hooked up a VGA monitor to the onboard VGA. Desktop came back rock solid. I've notified the vendor that the card is bad. We'll see how that goes. Thanks.