Dual Networks

We have a Intel P4 2.8 workstation with 80GB hard drive and 2 Intel Pro 10/100 NIC cards. On card is connected to our LAN (IP 172.20.22.201, Mask 255.255.252.0, NOGATE, DNS 172.20.20.2, 172.20.20.9). The other card connects to a DSL Netgear 7200 Series router that connects to a DSL line. The card is configured as (IP 192.168.2.27, Mask 255.255.255.0, Gate 192.168.2.1 (DSL DNS's)). We have a VB6 application that searches for a file on our LAN that has been desginated by hard coding the path inside it. We have also mapped the network drive so it would always be there. Every once in a while the application gives us the following error while accessing a file: "Run-time error '52': Bad File name or number" Pressing ok will either exit the program or continue on to the file. Here is what we have done:
Updated, reinstalled, and rearranged both NIC's

Hard coded the application to direct it to mapped drive.

Disabled and powersaving feature both on the NIC and the Motherboard.

Switched cables, ports, NICs, PC's and rebuit new PC's.

Removed any and all virus protection, firwall, 3rd party apps. Left with basic OS.

I suspect that when the app ( or any other function) tries to access the LAN it gets lost only to return after the error has been reported. I do notice that there are intermittant breaks in the network share access. Please let me know what you all think.

Thanks