disconnects

Hello!

We run pptp server for provide access for some resources inside our LAN.
And we found that if user generates heavy traffic, after some time 
connection is lost.
Server is ubuntu 12.04 with  1.3.4-5ubuntu2.
Clients are ubuntu and windows, there is no difference in behavior.
Looks like transmission just stops and I see in log:

Oct 10 09:49:36 inetgw2 pppd[1988]: No response to 20 echo-requests
Oct 10 09:49:36 inetgw2 pppd[1988]: Serial link appears to be disconnected.
Oct 10 09:49:36 inetgw2 pppd[1988]: Connect time 107.1 minutes.
Oct 10 09:49:36 inetgw2 pppd[1988]: Sent 1621850292 bytes, received 
139688426 bytes.
Oct 10 09:49:42 inetgw2 pppd[1988]: Connection terminated.
Oct 10 09:49:42 inetgw2 pppd[1988]: Modem hangup
Oct 10 09:49:42 inetgw2 pppd[1988]: Exit.
Oct 10 09:49:42 inetgw2 pptpd[1987]: GRE: 
read(fd=6,buffer=6075a0,len=8196) from PTY failed: status = -1 error = 
Input/output error, usually caused by unexpected termination of pppd, 
check option syntax and pppd logs


Could you , please, help me to find cause of this ?

Thank you!


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