Problems with MPEG Streamclip
 

Hi
I'm using MPEG Streamclip to prepare my vids for upload to the web.
I'm using FCP3 and converting to QT movies before running them through Streamclip to MPEGs
I get told there are lots of data-breaks and the MPEG goes blocky and sometimes the audio breaks up in places.
Anyone got any idea where I'm falling over?
Cheers