What you are seeing with the changing frequencies is something along the lines of Turbo modes. As Jeff mentioned, CPU's will adjust their speeds according to the work loads. Example, my CPU is rated at 3.4 GHz, with a 3.9 GHz Turbo mode. Be aware that the standard CPU coolers that come with most PC's or CPU's are intended for cooling at normal clock speeds and occasional Turbo mode. Permanent overclocking will always cause CPU's to generate excess heat, and that has to be dealt with using an aftermarket custom cooler. Most modern systems will throttle back the speeds when temperatures exceed a preset threshold, and you'll realize this as sluggish performance.
It is also a good idea to periodically open up your case and blow out any dust that accumulates, especially on the fans and heat sinks of the CPU.
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