Evaporator freezing up. My 15 Colorado work truck would do that on the run to El Paso, about four hours into the drive, no AC, turn off AC let fan run and melt the ice, AC works.

Never bothered to get it fixed. But usually when you are low on freon the evaporator freezing is a symptom. I know it's been checked, but being a nonstock application you might need a bit more freon in it. What do the pressures and ambient temps look like?