Grainy homemade yogurt

I know there have been multiple discussions of homemade yogurt on this board, but I can't find any that seem to address this particular issue. I have been making yogurt for a couple of years, with considerable success. The end result is not always entirely consistent, but always, with one exception, good (...the exception was the yogurt that would not die...it wouldn't stop culturing/fermenting, and blew the lid off its container in the fridge). I've played around with a variety of culturing methods, in summer it just sits on the counter of the warm kitchen, in winter I've done a crockpot (heated & turned off) a low oven(also heated & turned off) and a double-bowl system, with yogurt in the small bowl nestled into a bigger bowl of warm water.
My last few batches of yogurt are coming out grainy, as if there are small (really small) lumps in it. It is still very tasty, but the texture has lost it's creaminess. Anybody else have this happen? Any ideas? I drain through a clean dishtowel, but I don't think I'm over draining. I do add vanilla and sugar after I drain, but the graininess is there before I stir the sugar in. Help!