IE issue with proxy setting.

Hi all. My domain environment is configured with isa as a proxy server. All
my clients set their IE to detect proxy automatically. However i have many
users who travel overseas and whenever they are back to office they could not
access internet even though their IE is set to detect proxy automatically. I
have to clear the temp internet files, check and uncheck the "automatically
detect settings" chkbox a few times before they could access internet. How
can i solve this problem permanently. THks in advance.