To be honest, I reckon it is something deeper, cos it was acting funny the very first time i used Grab to take a full screenshot, made my iTunes go a bit whacky for a few seconds.
I've had a look in the console logs, and it gives me this for the times I've took the screenshot:
Sep 24 14:04:09 xxxxxxxx-computer kernel[0]: IOAudioStream0x4db4a00::clipIfNecessary() - Error: attempting to clip to a position more than one buffer ahead of last clip position (2,728)->(3,7bd).
Sep 24 14:04:09 xxxxxxxx-computer kernel[0]: IOAudioStream0x4db4a00::clipIfNecessary() - adjusting clipped position to (3,728)
Sep 24 14:04:32 xxxxxxxx-computer kernel[0]: IOAudioStream0x4db4a00::clipIfNecessary() - Error: attempting to clip to a position more than one buffer ahead of last clip position (2,71a)->(3,7ec).
Sep 24 14:04:32 xxxxxxxx-computer kernel[0]: IOAudioStream0x4db4a00::clipIfNecessary() - adjusting clipped position to (3,71a)
Sep 24 14:04:33 xxxxxxxx-computer kernel[0]: IOAudioStream0x4db4a00::clipIfNecessary() - Error: attempting to clip to a position more than one buffer ahead of last clip position (4,73a)->(5,7c8).
Sep 24 14:04:33 xxxxxxxx-computer kernel[0]: IOAudioStream0x4db4a00::clipIfNecessary() - adjusting clipped position to (5,73a)
reckon there might be a conflict somewhere? from my experience of chasing down issues on XP, when the sound would skip while another task was being executed it usually meant it was a shared resource, although it doesn't explain why the smaller screenshots are taken without any issue.
I've posted on the apple support forums, see if any mac geeks can shed a little light.