Streaming from WHS 2007 and Ripped DVD formating

This is my biggest gripe with the unit.  I have a ton of DVD’s ripped in file structure, and the machine stutters when playing them back.  Only way around this is to wrap them as ISO’s and then they play fine.  What gives?  This was the main reason I bought the unit; to stream already ripped DVD’s to my TV.  Until this gets fixed, I am not happy with the machine.