What cause’s a server to speed up and slows down?

I have a Dell 2950 Running windows server 2003. Ever so often it will speed up and slow down this is now more noticeable. I have ran many checks on it and all non conclusive. The event viewer shows nothing out of the ordinary but yet still this eratic behavior has me wondering if a failure is looming. the server is only use for as a print manager nothing else.On the network I have a DC and a Circulation on another server a total of 3 server any ideas or tips to point me in the right direction maybe I am overlooking something.I welcome any who run into this problem before. Thanks