Remove the round foil duct between the heater case and the grille. Just put the grille back on without the duct.
Removes a restriction allowing more air to flow. Low air flow can prevent the sail switch from activating.
Restricted air flow can possibly cause motor overheating then, tripping the thermal breaker inside the furnace.
Removed it on mine. No more problems since I did that to mine at elevations up to 10K feet.
This worked! I was up at Alta today skiing and it was nuking and cold. Worked like a charm. Thanks!