Did you make any changes to the dough after the 30-40 pizzas when the problem first arose, as by adding sugar or honey to the dough, or was the oil the only change? How much oil and flour are you using? Did you change the bake temperature or time after the 30-40 pizzas?
Are you using a gas oven and placing the stone on the floor of the oven? Are you using any cornmeal on your peel and, if so, are you cleaning your stone after each use to get rid of the burnt cornmeal?
If the stone is used properly, even if it looks grungy from baked-in fats, it shouldn't go bad.