TMG 2012 web error sometimes

Hi, I'm using TMG 2010 in my work network, we have 1 dc and 5 computers.

My problem is this: SOMETIMES I can't access to differents web (not anyone in special) from users computers or servers. I tested connecting the computers direct to the modem and the problem dissapears, also from the TMG server I don't that that problem.
It's just sometimes not always and if I refresh the page it connects sometimes.

Can someone give me an idea of what could be wrong?