CA/400 session dropping over VPN

We are in the process of attempting to convert our Frame Relay WAN to VPN.
LAN/WAN overview -
AS/400 iSeries 730, OS version 5.5 on host and client. dual NIC cards in 400
Lotus Notes version 5.0.10 running on Windows 2003 Server.
Cisco Catalyst 3524 XL.
Fortigate-60 firewalls (ICSA certified) with VPN support (IPSec, PPTP and
L2TP)
Four remote locations each with 384k fractional T1
Host location has full T1 (1.5Mb) of Internet connectivity
 
Issue -
CA/400 sessions at remote locations slow up drastically and finally drop.
Traffic is apparently continuing to stream from the remote clients to the
400, but the 400 is not sending anything back. The VPN tunnel is maintaining
integrity during the session drops.
 
Initial thoughts -
Does NAT address translation in a firewall interfere with the polling
process that allows the AS/400 to determine if a remote CA 5250 device is
still active?
 
Possible configuration problem with the port on the switch that the AS/400
is attached to, must be set up to allow broadcasts.
 
Is there a CHGTCP attribute that I am overlooking?
 
 
David Vega