I'm inclined to agree with some of the suggestions. I had a carb that acted exactly like yours does. It plagued me for months. I finally discovered that the needle was not totally free to move within the seat and would occasionally hang up and not close. Didn't do it all the time. The inside walls looked clean and the needle looked good. They simply weren't smooth enough. I solved the problem by polishing the needle with Brasso. I polished the cylinder for the needle with Brasso as well by using a QTip swab. I worked it for quite a while until the inside looked like a mirror. When I reassembled, it worked fine and hasn't had a problem since. Some rebuilding kits don't replace the needle and seat. I suppose that it is possible for a new one not to be slippery enough as well.

I definitely would not machine the halves. I don't think that would change anything but increase the risk of getting machine particles where they shouldn't be.

And the suggestion about checking the float is excellent as well though when that has happened to me, I couldn't get the blame thing to run at all.

Good luck,