Blockage in the air flow would be constant unless the spider is moving around.
I have seen is a loose orifice cause this, and it was on a newer unit.
Another area I would consider, since you rig is newer, is if you do not have many miles on it I would look at the possibility of oil getting to the burner. This could be from the gas valve manufacturing or the gas line.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles