VPN drop

We have someone work at home using XP connect to VPN
server(2003). But his vpn connection is not stable,
dropped 15 times per day, even redialed, outlook not
working. The only thing can do is reboot to get
everything back.

His IP address is 192.168.... even he connects to ISP
directly. ISP router support PPTP pass-through, otherwise
no VPN connection at all. Server is ok because other
people can connect to our VPN server for 20 hours without
drop. Anyone can help.