a lot of the CPU throttling going on nowadays is often from the high voltage requirements of modern CPUs like the Phenom IIs (X6's especially) putting a massive strain on motherboard power delivery.  when the mosfets overheat, depending on your motherboard they will either automatically throttle your CPU (ASUS), or potentially burst into flames (MSI).

i had an overclocked X6 that was constantly being throttled, despite ice cool CPU temps. i noticed the mosfets were getting so hot theyd burn your fingers.  some arctic silver thermal epoxy and a heatsink across the mosfets and no more throttling issues.

this never really used to be an issue.  however, with processors voltage requirements outpacing the average budget board's power delivery im seeing it crop up more and more and its one of those issues that people generally dont think about.  worst case scenario your board bursts into flame and potentially burns your house down.  if youre running a budget board with 4+1 phase power (usually micro ATX) then its worth your while to spend the <$20 and add some heatsinks to your mosfets.  if youre running an X6 this is something i wouldnt put off for a day.  if youre building a system from scratch, then spend more money on a non-budget board with 8+2 phase power and preferably heatsinked mosfets. 

everything comes together on a system build to ensure stability. most people just focus on the CPU, but ignore vital components like the motherboard and PSU and just get whatevers cheap. with the amount of wattage some processors are pulling this is not wise :(