I seem to be incapable these days of making a milk based cheese sauce, without the cheese separating as soon as I add to the thickened milk. I do the usual butter & flour cooked for a minute or two, then three cups of milk heated till just boiling and then thickened. The last three times I added the grated cheese (after I had taken the pot off the stove) it separated. I have good pots, so I don't get it. For the past year I have been using 1% organic milk - could that have anything to do with it (not that I could see how).