iMac freezing every 30 seconds or so

A strange new thing has started happening with my system.. Every 30 seconds or so the system seems to freeze for a few seconds then unfreezes and is fine for a period of time. For instance if I am playing a game that is graphic intensive the play will be fine till it hits that freeze point then all displays, response etc. pause. A few seconds later everything jumps to where it should be. The same thing happens if I am typing in Word for instance. I can keep typing and the system will catch up after freezing. If I watch the Activity monitor when this happens, the CPU usage seems to drop to 0 or very low then jumps right back up. Obviously this is an issue.

I have ran the TechTools Deluxe and it didn't find any issues.