I've been using a Hakko fx-888 soldering station for about a year now, and recently it starts to have a heating problem. Sometimes the tip works fine, ie. it's hot and the LED blinks to indicate it's heating but sometimes the LED is completely off and the tip is cold, even though soldering station still hums (it's on), and no matter how high I turn my dial, nothing happens. Buying a new soldering station is out of my budget at the moment, and I really like this soldering station. What can I do to fix the problem?