Why do my car run hot and when I slow down the temp. drops?

Okay I own a 1994 Cadillac Elderado. When I drive my car the thermostate temp goes over 208 and when it reaches 255 it digitaly reads egine hot cut off. After cutting the engine off, i check the coolant level and they are very low each time. Their is no leakage under the car. So where is all of the coolant going? Why do my car run hot? Oh I changed the thermastate.