You either have a deflected nozzles, need to do an alignment, or static.
Question: does it do it on both sides on bi-directional and one side on uni-directional?
Things to check:
1. Nozzle check test print will verifiy deflected nozzles.
2. Bi-directional alignment will fix alignment, or you may need a tech to physically align the Magenta print head.
3.We have had static problems like this on banner material only.
Symptoms: the print would start fine and after 4 ft when printing black the magenta would start overspraying. If we paused the printer and allowed the print head to make contact with the capping station, it would stop for another few feet. We solved the problem by turning off the heating and ac vent blowing on the printer. The vent was spewing charged air and led to static buildup.