Need to Renew DHCP License every few minutes

I've been using my current setup with no problems.
Out of the blue, my Mac Pro loses internet connection. At least, that's what it looks like. The browser keeps saying "Resolving Host" and just hangs up.
The only work-around has been to go to Network preferences and click Advanced and Renew DHCP License. This only works for a few minutes. Then it goes back to being stuck.

I've got an Airport extreme in the other room with 2 more macs running fine. This computer is a Power Mac running 10.6.8. It's hooked via ethernet into an Airport Express. I've also got an old Macbook downstairs that's working fine off the wi-fi.

Any ideas?

Your post was moved here as it has nothing to do with an iPhone and iPhone hardware. Please read forum descriptions before posting. Thanks.

Confusing. If it is a PowerMac it is not a Mac Pro as all models are Intel processors. And as it is running Snow Leopard OS X.6 it has to be an Intel machine.

However first up change the ethernet cable - they do fail on occasions.