Hi
I'm surprised that you would be losing count at as low as 200Hz, and after some google-fu found this article The overhead of Arduino Interrupts | Bill Grundmann's Blog which seems to indicate to me that 200kHz should be achievable, but that will rely on the interrupt function being simple and fast. Above you've not included your interrupt code which makes it hard to see where things might have gone wrong.
If by 200Hz you meant a higher frequency (kHz or even mHz?) then an external counter IC might be the best bet? How high do you expect your sampled frequency to go?
Remember when posting your sketch to include it inside CODE tags to make it easier to read (that's the # button above).
Cheers, Geoff