Ill get to the point, my screen blinks once, every 10 -20 minutes or so: its quite random when it does it to be honest. But its very annoying, and distracting at times. There's no loose cables, I have nothing but my ps3 (which is off) and cables and sockets near my screen. Im also using my 32" Samsung HDTV as my monitor via an ATi 7970 power color vortex II. Had no issues with the card in games so far at all runs fine, hasnt even over heated during either.
I do have a PSU which was cheap (big mistake) and im replacing tomorrow with an XFX 80 plus silver modular PSU, but im wondering if the cheap PSU cant handle my system and somehow that's effecting the graphics card, im not too sure to be honest. Just to note it sort of looks like the screen is readjusting, not like when you reset display settings, but very similar and i had ear phones in from front panel and i heard a fuzzy bleep when the screen blinked :S very strange.
Your PSU can't be the problem, if it couldn't deliver enough power your graphics would just shut down and crash your system. It could be a monitor issue, try running the same setup on a different monitor and report back your results. Could also be a graphics card issue, try updating and search forums for strange behaviors like that caused by 7970.
I dont have another monitor to do that right now, but i will note that in games i dont have this problem, and sometimes it doesnt occur at all. Later today im reinstalling windows 7 and starting fresh, since i have so much crap on my hard drive any number of things could be effecting this. My GPU temperatures are normal (29 degrees idle) so i dont think theres an issue there. I did have a 5770 previously, but i unistalled the drivers.
Another thing that might be causing it is the VGA cable. If you convert your white VGA cable (I'm assuming this is its name) to a blue one, or vice versa, you will have a pulsing effect while playing games or watching videos, even when not.
A second possibility is the presence of speakers. The presence of speakers can cause this effect, too, when they're turned on and even when not.
A third possibility is a bad wiring in the house. Try plugging in your computer monitor (and afterwards try your entire machine) into another power inlet while having no unnecessary appliances on.
Checking these 3 possibilities, if the problem still isn't fixed, it may also have something to do with your power supply unit in the computer, as mentioned by XYMan.