It could be the lock mechanism with it been so hot at times in japan it drys them out a bit.Take your door card off and give it a good seeing to with some good grease :surrender:
Start with the cheapest first so change the thermostat if that dont work.....as paul said id look into geting a temp gauge sensor.I cant see it been the headgasket
You must of spent a fair wack on your N/A sparky.....It looks the Dogs bits or should i say it did lol Also im not sure if its my computer but Lukes link is not working