This is all fine, it is because when the processor is doing little or no work it slows down to save power and reduce heat, and only ramps up to the full speed when it is something to do.
Try this tool: http://www.cpuid.com/index.php it will give you the specification of the processor showing it's full clock speed, and will show you in real time how fast or slow the processor is running. When you do something that means the processor has something to do you will see it speed up.
With your Info I have tried opening huge applications like entire adobe package so the Processor started showing up as 2.67Ghz..
Thanks for this amazing tool link...