Beef Stroganoff problems, diagnosed

Check when the graininess appeared. If the sauce curdled the moment sour cream was stirred into very hot liquid, the heat shock or lack of tempering is the strongest clue. If it was smooth at first and split only after the pan returned to a boil, overheating after the dairy was added is more likely. Reduced-fat or fat-free sour cream is also less stable in this kind of hot sauce than full-fat sour cream.