PIX 501 dhcp client

I'm setting up a PIX 501 on a home network with a cable modem and cannot obtain a lease for the external connection's dhcp client. I connected the PIX to a switch along with one of the home pcs so I can capture traffic between the firewall and the cable modem. The PIX sends discover packets every 3 seconds, but never receives offer packets from the dhcp server. An ipconfig release/renew on the pc not connected to the firewall leads to a new lease immediately. If I connect the outside interface to the inside, it immediately obtains a lease from the dhcp server on the inside interface. I think I must be missing something obvious.