Linux PPTP server disconnects

I run a PPTP server (1.3.4) on a CentOS 4.4 behind a Cisco ASA 5505.  It 
is connected to a 45MBit/sec backbone.

Client machines are Linux, Mac and Windows, at various locations.  All 
connect and work fine as long as their local network is not too busy, 
but suffer from frequent disconnects (most for Windows clients, then 
Macs, least for Linux) when there is other traffic on the local network.

It appears that the problem is most severe if all client traffic is 
routed through the vpn, less severe if unrelated traffic (e.g. cnn.com) 
is not routed through the vpn.

Since the problem occurs with multiple clients at multiple locations, I 
think it is a server problem. Also, the same clients connect to other 
vpns (running Windows based vpn servers I think) and do not exhibit this 
problem.

Running wireshark on the client one can see that when the disconnect 
occurs there still is gre traffic between the client and the server, 
however, the server claims that the client disconnected.

I sometimes simultaneously run an ssh connection from the same client to 
the same server.  While the vpn sputter, the ssh connection is rock solid.

Searching the Internet it seems this is a common problem with Linux 
pptp, and I am wondering whether it is a flaw in the pptp server, or 
common misconfiguration errors.

Any insight appreciated!

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