It depends on equipment you have. At higher rate, the specimen and oven heats not on the same rate, oven temperature is always a little bit higher than sample. Usually each time when you changing heating rate, equipment must be re-calibrated.  If problem persists, it means that you're using too much material and/or too  high heating rate. Lower heating rate produces more accurate results. usually ~10 deg/min is enough to get reliable results.