Aroud here in Talladega we have a little troublemaker called a "dirt dobber". In the summer time if you leave anything with a hole in it open they can build a dirt nest as big as a hot dog winney in a day.
I ruined an engine rebuild in a VW once when the little devils built in the intake manifold and when I started it up it ran great for about five minutes and then started doing all sorts or weird things till it finally just quit. Pulled the plugs and dirt was in all the cylinders. Had to pull the engine out and replace cylinder, pistons and rings etc. I hate dirt dobbers!!!!
Could be something in your exhaust manifold. I doubt it being in the intake unless maybe a mouse nest???