|Sometimes they get changed by overheating issues. This is a safety issue that will throttle back the speed of the CPU to reduce heat build up.|
For laptops the speed will change with the weakening of the battery as well.
Open the computer properties and see what the registered speed is (R. clk. My Computer) If it shows that it is lower than what it should be then there is a possibility that there is a heating issue.
Change Is Good