I have the same problem Imac.
Imac
Running OSX 10.6.8
2.4 GHZ Intel Core 2 Duo
4 GB 667 MHZ DDR2 SDRAM
If I wait a couple of minutes it will start working again. I reinstalled FCP and there was no change. It also does it on my Macbook as well.
Jim Miesner
585-590-6776
Fruit Tree Studios