header glowing?

maybe a mouse found his way in and is wearing a fire retardant suit and blocking that hole?
Sounds like a blockage problem or for some really weird reason- its firing late in that cylinder?? Maybe the header is just "thin" there? Is it right next to another exhaust-like one of the middle set? Maybe you have an exhaust leak there and the other exhaust is "bleeding" into that pipe also- keeping it hotter. Hell- good question! It does sound like a blockage or thin wall problem.