There are two times this can happen. First, they can downclock due to heat. Pretty sure this is all models though I'm not sure GPUs do this. Second, they can downclock while idling so they don't cost you as much money or get as hot. All CPU and GPUs do this, its nothing to worry about. You do need to worry about the first case as it can damage your CPU. The second case is nothing to worry about as it will go back to the correct clocks once you place a load on your GPU.
So right now while your typing on your computer, your cpu is running slower than it's rated speed, as is your graphics card. When you do something cpu/gpu intensive, they speed up to their rated speed.
In cases like multimedia computers that the processor is really more than
is needed for the multimedia work (streaming video,music etc)
users will downclock the CPU to conserve electricity and reduce heat
Since modern CPUs are more than powerful enough for multimedia
use they can have some performance reduced
Primarily reducing heat in a multimedia setup is done since the least
amount of fans in case causing noise is desired
Also to get an accurate idle temp
Users will turn every fan up and open the case
then they will downclock and lower voltage to
get the lowest idle temp possible to calibrate
their temperature monitoring programs
If the idle temp with air cooling is below
ambient room temp then you know
that the monitoring program is inaccurate
I'm more curios to know about auto downclocking.
I'm soon to build a new Sandy Bridge based PC as a Gaming and media center, running on a 42' LCD TV and home theatre amp. Hot climate, room with no air con and someone mentioned to me in another thread about the cpu and gup downclocling themselves if they over heat.
Hence my above questions about thresholds and the like?