I bought the car used, after having it for about a month it started over heating, so i changed the thermo and the rad cap and it quit over heating. the other night driving home it started to putter and blow white smoke out the exuast like crazy at a low rpm or sitting still, when the rpms hit about 3 1/2 it would run fine and stop smoking. about 10min later it started overheating like crazy and covering the road in white smoke out of the exuast. Anyone have any ideas? the oil is dark black, and the coolant looks neon green still im pretty sure