Same issue with my ST. I bought a thermostat for about £5 from ebay, an OEM part too! East to fit, three hex head screws hold it in. Make sure you buy one to include a new rubber seal for the thermostat. Give the housing a good clean out too if there is any gunk in the housing. You will have to top up the water level too. You should find the temp needle sits in the middle of the gauge and stays there irregardless of heater, engine load etc, etc. You may also notice engine runs better and fuel economy improves as the engine management system takes engine temp and water temp into account. Mine feels much better now it is warming up properly.