Verify if the fan works either electro, but I think that yours is thermogutyer, answer me if it works and if the water level low or maintained, if there is any leak or something, if all this is fine, take out the thermostat and leave it like that, do not buy a new one only leave it without it, if after all this it heats up the same, take out a spark plug and be if it is also Emulled, just get the oil filling lid and see if there as a cream, either of these last two alternatives are serious, and you should change the cylinder head gasket, now if it is only on low and when you go speed you take the most caused thing is that it is only the fan. Chea and let me know so that you enjoy your vacation soon.