I recently replaced the ducts from my Suburban Furnace. Factory tin-foil wrapped on a spring duct was kinked, had way too many 90 degree bends, and one duct was twice as long as it needed to be.
Definitely check for plenty of air flow from ducts when furnace is running. The more the better. If the heat exchanger isn't cooled enough the exact symptom you describe will occur.
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2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan