linode PPTP VPN setup, plz help

i have already setup the pptp VPN server on my linode, based on centos X86.

and i can connect vpn from my Win7. but after 30 -4 0 mins vpn disconntected automaticly, when i try to reconntect to the server. i got the 800 error.

i have to rebot my linode to get my vpn runing again. anyone have any idea? how to slove this problem? many thanks for the help.

by the way, i am a noob for coding stuff….