You can try calibrating the altimeter. And perhaps the opening of the altimeter pressure sensor is clogged with dirt. Try letting the watch for around an hour in luke warm water with a but of liquid soap, and then dry it carefully, blowing out the water from the pressure sensor oriffice. You should clean the watch regularly, anyway. Look at the opening with a magnifying glass and check wheter there is still not some dirt, but avoid pushing any sharp tools into it - there is a risk you could damage the sensor.
