No heat, overheating

Vehicle temperature gauge shows overheating (all the way over), warning lights and alarms come on. No heat, just cold air coming out of vents. This happened three times now, in a very cold winter, after a few days sitting idle. Always within a few miles of leaving my home. The first time it happened, I had it towed to mechanic who replaced thermostat. Happened again about two weeks later. I did not run for a few days, and I knew it was going to happen because nothing but cold air coming out with heat on, and rapidly rising temperature gauge. Was able to drive directly to mechanics garage. He put it on computer, checked it himself, drove it, etc. Nothing! A few weeks later, same exact scenario! And during a blinding snow storm I let it cool down, after checking radiator, which just like the other times was cold. Turned car off, gauge quickly started to drop with engine off. Did this three or four times before temperature gauge stayed at normal level and heat came out. Was able to drive to destination in snowstorm, about four miles and return home without further problems! Drove it the next day, no problems. This makes three times total (first time thermostat was replaced). With is going on, my mechanic is going to look again tomorrow, because it is going in for an oil change, which is low but changed regularly every 3,000. Any help or ideas are appreciated.