Home network/Tor exit, finding the limiting factor

I'm trying to determine what is limiting the number of connections my home network is able to handle. My setup is as follows:
DSL internet service
ISP supplied DSL modem, with it's firewall and wireless components disabled.
Smoothwall with separate LAN and DMZ network. The DMZ is configured to function as a 2nd LAN. No interaction between LANs is permitted.
One LAN connects to my primary PC. This PC also serves as the Tor exit node.
The 2nd LAN connects to a VOIP modem and a 2nd PC, running XP.
For the last 2 days, the Tor traffic has been almost constant, running at the maximum average volume permitted by torrc, which is 50% of my tested upload bandwidth. The burst setting is 75% of the tested bandwidth.

Although the traffic volume has been nearly constant, the number of connections is fluctuating widely. When the number of connections peaks, my browser becomes slow and erratic. Connections start timing out. At first I thought the problem was the PC itself, and my old operating system, Kex modified 98SE. It turns out that on the other PC, the XP unit on the other LAN, connections were timing out there just as much. The VOIP modem is also missing calls. I'm convinced that the problem is a limitation on the number of connections that one of the devices can handle. The only things these all have in common are Smoothwall, the DSL modem, and the internet service itself. Is there a way to determine which is the limiting factor without substituting other devices in their place? I don't want to shut down the exit node in order to find out. I'm somewhat out of my element here.
Any ideas?

edited to fix typos