I was working with someone last night on some performance tuning and noticed that they didn't have a specific issue that was causing their system to slow down - it was a combination of things. As we were working, I found a bunch of things that were running on a fixed cycle. A fixed cycle is something that wakes up, runs, and goes back to sleep.
You'll see this if you use a lot of SLEEP or WAITFOR statements in your code and T-SQL, or too many Agent Jobs on tight schedules. While these things aren't bad in themselves, you have to make sure you really need that cycle. In this case, there were event-based actions we set up to accomplish the same thing, and the system picked up some extra CPU cycles and calmed down.