For ages my car has had a problem where it takes ages to heat up, then will cool down again if I get on the freeway or whatever. The needle never stays in the middle where it should, it is always wandering around on the cold side of the temp dial. Car never gets too hot. This is because the thermostat is stuck open right?

What should I replace along with the thermostat?

It seems like it's best to do the thermostat, thermostat housing, crackpipe and whatever else all in one go. Like it's not worth doing one, best to do them all at once. Oh yeah and the crackpipe/oilcooler hoses. It never ends...