Random Lagging that has just suddenly come on

Hey mates! I've had a strange problem. Recently while playing Blood Bowl my computer has been lagging out for no reason I can see. I've checked the windows task manager when it's doing this and nothing seems to be hogging up the power (In fact it says cpu usage is low around 20% when the game is lagging out) I fixed this problem by lowering the graphics seemingly but it should be able to run the game without doing this! In between the random slow downs the game runs like a gem with the graphics maxed out (it isn't that demanding a game)

Now I've got Test Drive Unlimited because I've been looking for a driving game that I can enjoy. I started playing it after installing it ect and now the exact same thing is happening with this game. Every few minutes the game will lag to the point where it is uncontrollable but in between these times the game runs just fine! My wife says that this has been happening in TF2 as well. This just started happening out of no where and I havn't had any new programs added to or removed from the computer that should be doing this. I updated my drivers, scanned for viruses and ad-ware ect and have no clue what is going on.

Has anyone dealt with anything like this before? Note that blood bowl worked fine for weeks before it started doing this (and I didn't install a patch or anything it just started doing it without warning)