WAN NIC can’t get IP address after installing ISA Server 2006

I install ISA Server 2006 on my Windows Server 2003 R2 Standard Edition.
My ISP assign IP address automatically.
First, it works well.
But after one and half hour, I think, I can’t get IP address from my ISP.
So, I remove ISA Server 2006 and reformat my Server.
Then reinstall and test. It’s OK.
A few minutes later, I face with the same problem.

So anyone please help me about it.
Thanks a lot.