i have a problem with stuttering games that occurred out of the blue: I can play any game, for example Skyrim, without any stuttering for a few minutes, after that it starts jerking. The severity seems to depend on the game and the environment, in caves it mostly runs smoothly. The problem startet around a year ago without any hardware changes. I own a Geforce GTX 460 with 767 MB Ram and the AMD Phenom II X4 945, 4GB Ram. Does it sound more like a problem of the CPU or the graphics card?
Since it's ~70% in Skyrim, it cannot be the CPU. Also, it's normal for a CPU to be on full load. 70% is not that high.
Try using GPU-Z and Task Manager in conjunction to see your GPU's temperature at the stuttering area, as well as the CPU usage and the GPU usage.
Also, try out SpeedFan.