Microsoft VPN keeps getting dropped almost every 10 mins.! Why?

Hi,

I have setup two VPN servers in my work environment. 
The two servers sit behind a hardware firewall and 
both machines are mapped to external IPs via NetScreen
25.

Each machine has two NICs and they both have Windows
2000 Server
installed (with all Microsoft's latest patches
applied). Internet
Authentication Service is installed and both machines
belongs to
a domain controller which provide authentication
services for these 
VPN servers.

Since both machines have identical setup (except IP), 
I will give you config of VPN Server1:

Two NICs:(with bogus IPs)

NIC#1Public:   192.168.1.90 mapped to 202.15.15.90
SubNet MasK   255.255.255.0   
Default Gtw   192.168.1.1
Preferred DNS 192.168.1.80

NIC#2Private: 192.168.1.92 
SubNet MasK   255.255.255.0   
Default Gtw(blank-per MS manual i found on the
Internet)
Preferred DNS 192.168.1.80

Users are prompted for password and authenticated 
with no problem but only stay connected for 
about 10 to 15 minutes then gets dropped 
eventhough they can reconnect with no problem. 
This problem persist for every user trying to VPN in.

Can someone tell me if my configuration is correct?
What can I do to rectify this?

Any help on this would be much appreciated.

Thanks In Advance,

-Exo

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