The only sure way to check for a defective modem is to swap in a known good modem and watch for the problem to re-appear.
If there is a problem with the 4100, usually it is due to a failure of the wall wart than the modem itself. That is more when there is a total failure than some intermittent failure.
Your download through put is lower than it should be on an Elite profile. I wonder if they have set your profile wrong at the gateway.
You might try posting in AT&T Direct: »AT&T Direct
to see if a tech there thinks it is due to your modem. Also you might mention your download throughput is below 5 mbps down.--
TCE Weather -- Mt. Rose Cam