How are you getting the values from ICB and sending them out the serial port? Unless you have some decoupling mechanism, the values you sending may be missed every few captures - according to your numbers, you should get a capture every 625uS - depending on your serial baud rate, you won't send too many characters within that period. If you have a queue mechanism to store the captures into and the serial task retreives these values, then I would think you may have a problem, otherwise the captures are happening faster than you can send them out - thus you may be missing some captures.