Please Help me in VPN Setup and a good VPN hardware recommendation

I work for a small retail business and recently installed IBM 4690
Point of Sale system. We also installed Verifone 7000 signature capture
credit card readers. VPN is currently tunneled through NetGear FS318.
We have been noticing some problems with the response not being
received at the registers and the reader. Somewhere data get lost or
packet loss. Here is my setup:

DSL via T1
|
Switch
--------------------------------------------
| |
VPN ZyXel Router
Static IP Static IP
| |
---------------- Switch -----------------
| | | |
IBM IBM SBS 20
4690 AS/400 2003 PCS
|
8 Registers DHCP to IBM 4690

NetGear has its own WAN and LAN addresses and exclusively used by IBM
4690
ZyXel also has its own WAN and LAN addresses and primarily used by all
other PCS.

We have experienced data loss either by NetGear or in the above setup.
Do I need to replace VPN? or any other solution might help. Thanks.

.