Airport card and ethernet show connection but I cannot connect to network

Question

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.

Answer 1

You have not detailed the router being used..
Please tell us if you are using an apple router and which one.. exact model will help.
Is the modem also a router? And wireless?
Try clearing out all the old settings from the wireless.
Reboot and do a clean setup.. certainly if there is an apple router it should have a factory reset and a new clean setup.
For ethernet it doesn't work like wireless.. it is based usually on dhcp.. things can go wrong though.. test with a static ip set on the computer.
Sometimes just age catches up.. old computers and routers and cables.. you need to test with new routers, new cables and new computers.. to locate where a problem is actually occuring.