Try calibrating your thermometer. Boil some water on the stove, and when it's briskly boiling, put your thermometer in and see if it reads 212 F. (If you live at very high altitude, the water will boil at a slightly lower temperature, so you'd need to look that up.) If it doesn't, you can still use it, but you need to make a temperature adjustment. For instance, if your thermometer reads 200 F in boiling water, then you know the actual oven temperature is 12 degrees higher than your thermometer says, so you'd adjust your oven temperature until the thermometer reads 263 F, and then you'd know the actual temperature in the oven is 263 + 12 = 275 F.
I also cover my clay with a foil tent, bake on a pizza stone to provide a more even temperature, and preheat the oven to avoid temperature spikes. HTH - Beth