Sounds like a classic case of "carburetor icing" to me. Ice can actually form in the venturi, especially during high humidity & even with ambient temperatures in the 80's or 90's. Small aircraft engines have "carb heat" to prevent it during landing & take-off. It usually happens during a decent to land when the engine is throttled back, not producing much power, so the engine cools off enough to allow the venturi to ice up. That's why you always land with carb heat on. Several years back I owned a Toyota which did it occasionally.