I found that using cheap lamp cord I was getting line loss and it was allowing the end tape to be lower temps than the first ones on the run. I was getting that kinds of temps top to bottom. I also found that the top due to thermal gain was hotter. If you are using lamp cord try re-wiring the with 2 strand heater cord. The other thought is to take the first off the wire to the bottom and last off the wire to the top. If the first is hottest and the last coolest the heat rising to the top drawers might off set some of the difference.