the e71 comes with a light sensor, meaning when in dark conditions, the light will brighten, in brighter conditions, the light will dim. Sometimes even if you're in the same amount of light, the way you hold or handle your phone may also interfere with the light sensor whereby your finger brushes and closes the light sensor which is located to the left of the earpiece on your phone. You can adjust the sensitivity of this light sensor by going to Settings - General - Personalisation - Display - Light Sensor. Adjust accordingly and no there isn't anything wrong with your phone, just one of the functions of the phone to save energy consumption.