Unfortunately, by cranking up the heat, the barcodes may look good to the human eye but your barcode scanner may not like them. This is because the width of the bars tends to grow leaving less white space which throws off the bar to space ratio.
SOLUTION: Turn down the heat until the image starts to fade out and then slowly increase the settings, one level at a time. Print a few labels at each setting and test them with your verifier. The key is to STOP at the lowest possible setting that gives you a good ANSI grade reading. Not only will you achieve more consistent barcode scanning but you will also prolong the life of your printhead.