car overheats and gets stuck at 20 miles and after a minute or two it runs fine

Is the engine boiling the coolant where it leaks out the reservoir? if so most likely thermostat.If its not boiling over could be a sensor problem. Try to determine which one its doing before you bring it to shop it will help them get to the root cause faster which will save you money in the long run.Give them as much info as you can like,does it happen every time,does it do it on hot days only,does it do it when driving freeway or in town driving.