It doesn't fall off as much as it 'snaps' into place and it only seems to do this when I put extra pressure on the pedals to go faster. I've looked at the chain an it doesn't appear to droop so I can't imagine it's loose.

Here's a picture anyway as I am no expert.
http://i53.tinypic.com/161b3me.jpg

The 'snapping' effect I get when I apply pressure doesn't seem to happen any more or less on any secondary gear, so I can only assume it's something to do with the primary gear or the chain itself.

Any input is appreciated.
It's a specialized bike, 24 gears if this helps. Not sure of the model.