VPN – Windows2000Server RRAS

Hello. I set up a VPN Server on Windows2000 using built-in RRAS. The primary thing is to connect our branch offices. Remote users have WindowsXP.
The weird thing is that the VPN works but when its a little idle users can’t open our working application in their browser. So i noticed. I ping them when they connect (passes OK). But after a hour (more or less) ping says Request Timed Out. But RRAS shows users that they are connected. So they need to reconnect to continue working.
I checked for firewall. No firewall at Remote site. Any ideas.