New build - display stuttering on desktop

Recently finished my first computer build, and after some initial problems with no boot (had a case fan connected to both the mobo and the PSU) and no display (GPU apparently not inserted just right), everything seemed to be working fine. I'd noticed the mouse jumping around/stuttering a bit, but I assumed it was due to the fact that I was using a rather old laser mouse on a somewhat shiny surface. I was wrong.

It's not just the mouse pointer--I checked by playing a video, and the entire display stutters. Note that I'm not getting any error messages at all, nor any kind of freezing (but I haven't tested it with any games yet, either). The problem seems to become more pronounced when multiple windows are open, and particularly bad when a notification pops up on the taskbar.

Anyone know what could be causing this? Is it the GPU, CPU, some OS setting I'm not aware of? I'm not sure how to figure it out, and google wasn't very helpful (mentioning framerate floods the results with game-specific forum posts).

CPU: Intel i5 3750k
Mobo: Gigabyte GA-Z77X
GPU: Gigabyte HD7870
Memory: 2x4GB G.SKILL
PSU: Corsair HX750
OS: Windows 7 Home Premium 64bit

I posted this on the New Build board as well, but upon further review, this board may be more appropriate.

UPDATE: Yes, it was a bad GPU.

SECOND UPDATE: No it wasn't. I had the power supply cables both connected to one slot on the PSU via the adapters that came with the card. This wasn't right, apparently. Not that the thing came with any sort of manual to tell me otherwise. Live and learn, I guess.