One possible fix for internet connections that drop out

If you search the Apple Discussion boards you will see a lot of discussions .
they're all about Airport and Ethernet internet connections dropping. I think I have figured out a solution to the problem (I use 10.5.3), as I was having it as well. 

With certain routers, OS X will choose to use the address of the router as the DNS server's IP address. Therefore, it will first try and contact the router to get DNS info, and only after a while will it find it's way to the net. I had a new MacBook Pro with this issue, as well as a new iMac. The latter fortunately showed the wrong DNS server IP (grey fonts), but the Macbook didn't. 

The solution is to go in the Network System Preferences panel and select 'Manually' instead of 'Using DCHP'. Then fill in the IP address, subnet mask, router address, and DNS server address. Click 'Advanced' and then go to the 'DNS' tab to verify whether the correct DNS IP has been entered, or to enter multiple DNS IP addresses. While in the Advanced section, make sure that under the 'TCP/IP' tab, 'Configure Ipv4' is set to 'Manually,' and 'Configure IPv6' is set to 'off.' 

This should do the trick for most people. So remember, if one has a dropping internet connection and no DNS server IP has been entered, the OS may be trying to contact the router rather than the DNS server of the ISP. Therefore, the correct DNS IP addresses need to be filled in.