What the seller told me about the head being milled was that when the plastic thermostat housing exploded, the engine overheated. He felt like it was not running well so he did a cylinder compression check and after that decided he needed to get the head milled. I have driven behind it while my wife drove it and did not see white smoke. I have not, however, done that when it starts to run hot. I am also not sure about when the thermostat was replaced. If he did it when he did all the other work on the engine it has only been 3 months. I know for certain that there is a blockage at times, which is why I asked in my original post if the thermostat could be sticking when the heat control is turned on. That doesn't seem likely to me, if I understand thermostats correctly, they open at a designed temperature regardless of anything else. Thanks for the replies.