TMG 2010 Problem

HI

i have tmg 2010 in my network and i have a problem with it

my tmg has 2 interfaces Internal and External

when i set the dns on Internal interface clients have problem to load some of Internet URLs and when i set the dns on External clients does not have any problem

so many of documents emphasize that for best performance do not set the dns on external and set it on internal

can anybody help to fix this????

thanks for helping

alfONso