Two Trixbox boxes on same subnet causes modem/router crashes

I've spent countless hours scouring the Interwebs for even a mention of this problem to no avail, so posting here is a close-to-last-resort for me.

I've personally had a PIII box running trixbox 2.4.2 (kernel 2.6.18-53.1.4.el5 [SMP]) for about a year with great success. I have one extension on site and another offsite at my home, both working wonderfully. In fact, I've had so much success, my father asked if I could build one for his business. I estimated smooth sailing. I was wrong.

I built the box from scratch using an Intel Core 2 Duo processor in a barebones box from Newegg. I had a very difficult time getting the onboard LAN recognized by the Trixbox installer (2.6), so ended up installing an old DLink card that was recognized right away. So I proceeded to install Trixbox 2.6 on this box.

We are both using Voicepulse for differing services, but both for outgoing. Mine is set up using their module, his is manually set up through FreePBX.

My father has 5 static IPs for his use and I'd been using one of those (let's call it *.81) for the year. In the setup of the Netopia modem/router, I had an IPMap configured so that a request for *.81 went to internal *.40 (my asterisk box). Once I added a second IPMap (external *.80) to forward on to my father's box (internal *.50), the modem reboots itself every 2-4 minutes. Ergh.

Through some trial and error, here are some things I've found:
1) It doesn't matter which box is plugged in first -- it works fine -- UNTIL the second is plugged in, then the reboot fairy visits.
2) I installed 2.4 on my father's box and things worked peachy for about a week, then the reboots started again, seemingly out of the blue.
3) When both boxes are plugged in, I can issue 'service asterisk stop' on either of the boxes and the reboots stop.
4) When my laptop is set up to receive traffic from the second IPMap, things work fine.

I talked to our ISP's Tier II support for about an hour without any results. While I was on the line, he actually got a Netopia tech on and we conferenced for another 30 minutes. Neither of them had ever heard of such a thing. We've had 3 techs come on-site and they all found nothing. We've even been through 3 different modems to rule out the modem itself. I've tried a different NIC card (NetLink) in my father's box. I've tried different ports in the modem. I've tried different Ethernet cables.

I am out of ideas of what else to try. Has anyone experienced this? Can anyone think of what else to try? What else to ask?

Thanks for any and all help. This has turned into a multi-month project because of this issue and I'd like to get it resolved.