I have this issue as well, and it's just because of what has been said above- the chainring just isn't perfectly round. It's an Origin 8 crankset and while not dirt cheap, it wasn't high end by any stretch of the measure.
My tightest area of the rotation is when the crank arm is passing 1-2 o'clock. When I am adjusting my chain, I make it so it's just barely not too tight it will bind at that position, then the rest of the rotation the chain has acceptable slack (but not so much it could just bounce off). Has worked great since I built the bike 4 years ago.