I had exactly the same problem with GA770TA-UD3 and Athlon 2 x4 630, that is until earlier today. And let me tell you, I would never have guessed the culprit to be THE GRAPHICS CARD (GeForce 7950GT)!!!
The 7950GT has 3 outputs, 2 DVI and the useless S-Video output.
When the monitor was plugged in the LEFT DVI connector on the graphics card, all of the symptoms you mentioned above were present.
When the monitor was plugged in the RIGHT DVI connector, the system was and still is working FLAWLESSLY, no matter what settings in the BIOS I changed (well, obviously, not EVERY setting, but you know what I mean)!
For the life of me, I cannot link the cause and effect of these two, but after two days of trying to make this work, I won't bother. It works!!!
(Oh, and the 7950GT works well too in the system. Funny enough, the 7950GT's both DVI outputs work fine on my other Gigabyte MBO with Intel chipset and CPU. Go figure!)
Anyway, hope this helps someone...cheers!