Last summer I had issue's with vapour lock. I redid my fuel supply lines leading to my TPI before the problems started. The fuel supply line now comes up from the frame rail,makes a 90* turn and runs parrallel to the ground for about a foot,then up to the fuel rails. After the vapour lock started I insulated the fuel supply line with a reflective cloth insulation...did't help though. The times it did happen the ambiant air temp was in the 90-100* area and the truck was pulling a loaded trailer. Could the fuel be vapourizing in the fuel rails?