Lose wireless network (Airport Extreme) when 2nd computer connects

I have a 1st generation Airport Extreme 802.11n that has worked flawlessly with a 2007 iMac (24-inch). However, when we bought a MacBook Pro (15-inch) in 2010 and began using it on the network, the iMac would not be able to connect to the network after the MacBook had been used on the network. Unplugging the Airport Extreme and restarting it would resolve the problem -- until the next time that the MacBook had been used on the network. (Just having the MacBook on was not a problem, only if it used the network to connect to the Internet.)

This does not happen all the time, but often enough that having to unplug and replug the Airport Extreme becomes bothersome. Recently, we have another MacBook and a Windows computer in the household, and they are able to connect to the network with problem -- and without causing problems with the network for others.

I updated Airport Utility on the iMac and the troublesome MacBook to the same version, and that seemed to resolve the problem -- for a while. Both computers are updated to 10.6.8.

Recently, the problem began happening again.

The Airport Extreme has firmware version 7.3.1.

Has anyone experience such a problem, or can anyone suggest whatmight be casuing it or how to resolve it?