Configure IPv4 'Manually' keeps changing itself to 'Using DHCP'

I configured my router to use Address Reservations -- that is, to assign specific IP addresses to specific wireless cards in the various devices on my home network: computers, TiVos, my iPhone. Each individual device is then configured manually with its corresponding IP address. This works perfectly on every device -- except for my Mac Pro running OS X 10.5.8.

In the "Network" pane of System Preferences on the Mac Pro, I select AirPort from the left panel, and then click the Advanced... button. In the next screen, I click the TCP/IP tab, and select "Manually" from the "Configure IPv4" pop-up menu. Then I enter the appropriate IP address, Subnet Mask, and Router IP. "Configure IPv6" is off, though it doesn't seem to affect the problem one way or another.

Here's the problem. Once the Mac Pro has connected to the network, it has a habit of changing the "Configure IPv4" menu setting to "DHCP" -- all by itself. I'll go back and look at it, and discover that this has happened. A couple of times, the router -- despite its Address Reservation settings -- has assigned a different ID to the Mac Pro.

I know... that's a separate issue with the router. But the point is that the Mac Pro is changing the setting by itself. Any idea what's up with this??

Thanks,
DM