Why would my framerate lag when only 10-14 of my CPU is being used?

Is there anything else besides high cpu usage that would cause the framerate of a swf to lag?

I’m working on a game that is set to 60fps so the animations are super smooth, and I am on good, fast computer so even with tons of animations going my cpu usage never goes above 14% usage, and only about 1% of the systems memory is used. Despite this, sometimes the framerate goes down to the low 20’s.

Any ideas?