Strange DHCP Issue?

I have two Macs hardwired behind an Airport Base Station (snow) and one served wirelessly. Recently the wired Macs began to access the net very very slowly. The initial connection, either for surfing or sending e-mail, could be delayed by as much as ten seconds. Once I connected to a domain, it seemed to speed up and subsequent pages loaded more or less normally. Oddly the PowerBook's wireless hookup was working fine.

I've fixed the problem, more or less, but I don't understand exactly how -- or why I was having a problem in the first place. Taking a wild guess, I reset the network settings for the two wired Mac to DHCP. Previously, they'd been set for DHCP with Manual Address. (The PowerBook had always been set for DHCP, which I guess is why I never had a problem with it.)

So, why should I have any issues using DCHP with Manual Address? Why should I start having a problem with this setting on two Macs that had been configured this way for years without issues? Could my ISP have changed something? Could I have screwed up my base station settings somehow?

To make matters even a bit more confusing, now, when I reset to DHCP with Manual Address (yes, I typed in the 10.0.1.x number), I get no net connection at all!

Suggestions...??