You say it seems to be intermittent. This happens for me if I haven't used Exposé for a long time. Then when I pop it up, it's as though it's reading the hard drive pulling Exposé into RAM. In cases like that, I've noticed that OS X interprets short keyboard entries as long ones due to that extra delay in reading from swap. Thus, Exposé only pops up for a fraction of a second. (The same thing would happen if you held down the F3 key for a second or longer, then release.) On the second try, Exposé is fresh in RAM and comes up like it should. Is this what you're experiencing?
I could be totally wrong about what's happening, and this is just my interpretation. Sorry, I have no fix for this.