Lag sometimes but not usually

Recently I've noticed a lag issue occurring at least once or twice a day, when I play a game.

Sometimes I start my game up (Urban Terror) and I run into some FPS lag. Usually I get a steady 125 frames per second (which is the max FPS the game can take) but on these certain occasions my FPS is considerably lower in most places in maps in the game.

After I see this issue I log off my PC and back in again, open up my game and it runs fine, no lag issues at all.

Does anyone have any idea why my computer is causing me to lag on occasions?

Just to clarify, the game in particular has nothing to do with it. Something on my computer is causing this.

bumpbumpbump

mpbumpbumpbu

Stop bumping... you're making me dizzy! :-)

How did you come to a conclusion that it's a computer issue and not the game? Are the delays happening elsewhere?
Give a little specifics about your hardware and software you are using. Also, has this always happened or just started happening?