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Question: Overheating Problem

Hey i have had the absolute worst luck with this car. It ran great for the first while that i had the car. I loved it and with only 25,000 miles i was looking forward to enjoying this fun car. I bought it used and it turned out that there was a hairline crack in one of the radiator hoses. All of the antifreeze leaked out and of course my daughter didnt notice the temperature gauge. I'll spare you the story but since that incident i have replaced the hoses (to prevent future leaks), the water pump, the radiator, and the thermostat. It still overheats. I believe the problem is at the thermostat because the hose going into the engine is of course very hot with the hot antifreeze in it and the hose going out of the engine is cool so obviously the antifreeze is not flowing out of the engine to cool back down. It's very strange but i can sit in the car and idle it at 3000 rpm but as long as its not in gear it will not overheat. After the first few miles of driving it then the engine will overheat. If i then pull over and shut the engine off and then restart then something seems to reset because then i can feel the antifreeze flow through the hoses and the engine temperature will return to normal. I dont know what could be wrong witht he thermostat because its new but that seems to be where the problem is coming from. I have taken the car to Mazda and had a block pressure test on it and there are no leaks in the engine becasue at first i thought either a cracked head or a bad head gasket but all the tests came back with normal compression. But somehow i think that an air pocket is getting let into the cooling system and when it gets to the thermostat it causes it to stop because the thermostat is water/heat activated. I am looking for any advice you may have and also am wondering what would cause air to get into the cooling system only when it's in gear, and not when it is in park. Also i think that by shutting the engine off it of course stops the pressure and flow of the antifreeze which causes it to settle and move the air pocket out of the thermostat. I cannot think of any more solutions or any more parts to replace that could be out. Thank You