Why does my Intel CPU underclock itself automatically?

Question

I have an Intel i3 350m and a Core 2 Quad 8300. The 350m is 2.2ghz, while the Core 2 Quad is 2.5ghz. With both, while looking at CPU-Z, they underclock themselves at seemingly random intervals (even while running games and stuff), then go back to their full speed. Why does it do this? Refer to core speed section below:

Answer 1

This is the Enhanced Intel Speedstep Technology at work. You can read more about it on Intel's web site at http://www.intel.com/support/processors/sb/CS-028855.htm.

Answer 2

There's a setting in Windows Vista/7's Power control panel with something to the effect of "CPU Power Management" that might be affecting this as well.