White smoke

I bought this car from a tow yard four days ago. I noticed the speedometer did not work until sixty to sixty five mph, and the temperature would run cold on the freeway, but the heater still worked. So my fiancé replaced the thermostat. Directly afterward the car ran hot while idle and there was some bubbling where the coolant is? Also, there was white smoke and the heater stopped working. He turned the car off and back on and everything went back to normal. The temperature was steady, the heater worked, and the car drove smoothly. However there is still some smoke, although it is raining outside so fairly cold? What could this be, if anything? Could it be from needing some sort of system flush? Or a faulty head gasket?