Wifi connected but won't pass traffic

My iMac all of the sudden stops passing traffic over wifi. It will however still show that it is connected to my wifi with full strength.

I'll notice the problem occurs because whatever web page I'm attempting to load will time out. I will then open the terminal and try to ping the router I'm connected to, which will fail. I won't be able to ping anything else on my network, however the wifi will still show connected. All other wireless devices in the house will continue to work just fine.

Here is what my wifi utility will show while the problem is happening:

If I turn the wifi off, then back on, the problem is temporarily solved, until it happens again, which is quite random.

Here is what the wifi utility will show when everything is working properly:

I have tried changing routers and used everything from Apple Airports to Asus to Netgear. The problem occurs with any of them. My other computers, both apple and non apple do NOT have this problem. I've taken the iMac in to the apple store, where they replaced the wifi card and antennas, but the problem still occurs.

I've wrote a script to help me troubleshoot this problem. I will run it in the future when the problem occurs:

date >> ~/wireless_tests.log

echo ' ' >> ~/wireless_tests.log

echo 'netstat -i' >> ~/wireless_tests.log

netstat -i >> ~/wireless_tests.log

echo ' ' >> ~/wireless_tests.log

echo 'airport -I' >> ~/wireless_tests.log

airport -I >> ~/wireless_tests.log

echo ' ' >> ~/wireless_tests.log

echo 'airport -s' >> ~/wireless_tests.log

airport -s >> ~/wireless_tests.log

echo ' ' >> ~/wireless_tests.log

echo 'tail -20 /var/log/wifi.log' >> ~/wireless_tests.log

tail -20 /var/log/wifi.log >> ~/wireless_tests.log

Can anyone think of anything else useful I can add to my diagnostics script?

Does anyone have any idea why this problem is happening?