Airport and ethernet show connection but I cannot connect to network

[nat-x]

I have an iMac where I can connect to the internet within 10-15 seconds of turning on airport or plugging in cat-5 wire to use the ethernet connection. After this time frame I must turn airport off and then on to regain the connection. Or, if hard wired, I must remove the cat-5 and plug it back in. I have seen many posts similar to this on forums. However, all of the posts pertain to a wireless connection only. I have tried most all of the solutions people have offered including zapping my PRAM. Unfortunately, I have had no luck with any of them.
Three things to note:
1. I believe this problem occurred after a power loss at my home. No power surge - just lost power. I can't be certain on the timing but thought this should be mentioned.
2. I have upgraded from 10.5 to 10.6 to try to rectify the issue - didn't work.
3. I have switched from comcast to AT&T - different router/different connection/different network preference - and nothing changed.
So, the problem is not with the connection itself as I am able to connect for a few seconds. It doesn't pertain to the OS as I have upgraded and still no fix.

Thanks in advance for your help.

 

[Brian33]

Wow, that is weird. I assume by "turning on/off airport" you mean the Airport card in your iMac, and not an Airport Express or Airport Extreme Base station router.

I guess you have an older iMac (as do I), but that shouldn't be the problem. The fact that the same problem behavior occurs with both Airport and Ethernet seems to argue against a hardware problem.

I think that leaves the OS as the most suspect, in spite of your upgrade. Can you have a friend with a laptop bring it over and see if his/her machine has the same problem?

The only other thing I can think of is to look in the system logs for clues. Open /Applications/Utilities/Console.app. Select View-->Show Log List, and then make sure you select DATABASE SEARCHES-->All Messages.

You can filter the displayed log entries by typing "network interface" or something in the filter area.

 

[nat-x]

Thanks for the response Brian33. You are correct that I was referring to the airport card in the imac. The imac is a Core Duo. So, it is a bit old. After filtering with the word "network" there are many submissions that state "network configuration changed." What exactly am I looking for in the console?