Intel Q8300 not going rated speeds

[Shadowex99]

My Intel Q8300 has not being going rated speed for a while now and I wanted to know why. It idles at around 1.95 - 2.10 ghz but when running games and prime 95 stays at 2.20 - 2.27 ghz 90 - 91% in task manager. Multiplier is locked and this cpu is 5 years old still running strong on Windows 8.1. What do I do?

EDIT: Its prebuilt with an added GPU

 

[Tradesman1]

You don't mention what mobo but will guess the FSB is not at 1333 which is what the CPU needs to run to full spec of 2.5