Q: Internet dropping Internet connection 15-20 seconds after connecting; other devices still connect to the internet

This is the Mac Pro desktop forum. I requested your post be moved to the PowerMac forum.

https://discussions.apple.com/community/desktop_computers/power_mac

Might try this...

Make a New Location, Using network locations in Mac OS X ...

http://support.apple.com/kb/HT2712

old... http://docs.info.apple.com/article.html?artnum=106653

Hi, this has worked for a few...

Though all of these steps may or may not be needed, I'm including them all.

Make a New Location, Using network locations in Mac OS X ...

http://support.apple.com/kb/HT2712

10.5, 10.6, 10.7 & 10.8…

System Preferences>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.

10.4 instructions...

Is that Interface dragged to the top of Network>Show:>Network Port Configurations.

If using Wifi/Airport...

Instead of joining your Network from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed.

Then... Try putting these numbers in Network>TCP/IP>DNS Servers, for the Interface you connect with...

208.67.222.222

208.67.220.220

Then Apply.

(There may be better or faster DNS numbers in your area, but these should be a good test).

Click OK.