It defiantly sounds like it's the sail switch problem. Not necessarily the actual sail switch, but the pressure the furnace is blowing the air to activate the switch. You should check around the furnace itself, specially where the blower fan is, and use a small paint brush and vacuum and suck out all the dust around the motor and the fan, belts, anything that moves. Then maybe a drop or two of oil where things move and touch each other. It does not take much dust to slow the fan down, as you stated.
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2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually
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