RDP disconnect in Windows 7 seems to be Firewall related

I have some users who RDP to their desktops from their home PC's (CISCO VPN Client connects first) to work out of hours and also connect to their desktops whilst in meetings etc (from within the corporate network). When we were on XP this worked flawlessly with users staying connected as long as they needed to.
Now that we have implemented Windows 7 and a Firewall policy it seems that they can only get 4 hours at the most but can disconnect within 5 minutes. If you turn off the Windows 7 Firewall it works fine again. There seems to be nothing really in the policy that would seem to cause this. RDP is definitely allowed.
If anyone can think of something it would be most appreciated.

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