Web based outlook access timeout issue

Dear Gents, Let me brief you the setup we have and later I'll describe the issue: I've two internet connections from two different ISPs in Saudi Arabia, and we use web based OUTLOOK access to our servers in US & UK. the issue is happening ONLY with one of those two ISPs which is kicking out my outlook session randomly in between 5 to 15 minutes. while with the other connection life is good and everything is going smoothly with NO interruptions.

Any advice?

Thanx, Amin