Erikka is on target. What you are most likely seeing is the result of the banner material being slightly squished and flattened by the rollers. This is causing a microscopic thickness change in the media and changing the printhead to material distance. This results in a tiny variation in the bi-directional alignment in the areas where the rollers are, the result being a tiny variation in the way the print looks in that area. If you repeat your print with the only change being printing in uni-directional mode, you will probably see it minimized or disappear. Of course this isn't optimal for production. The key, as many have said, is to use only the outer rollers for printing. Using the inner rollers for cutting only will help with accuracy, especially on longer runs. Reducing your heat will also help, but isn't the best answer. Not sure which model you have, but a trick with heavy rolls is to remove the front media support shaft in the back of the machine and run the rear shaft (the one that has the weight brake) through the core of the heavy material, like a paper towel holder. This reduces the force required to pull the material through the printer by about 80% (a guess), resulting in less stress on your feed motor, less media feed variations as the roll gets lighter and more consistent printing through the roll.
Regards,
Jim