I have a 626 that overheats when accelerating or maintaining a speed above 30MPH, once the car is stopped the temp returns to normal almost immediatly it's like the temp gauge is directly tied into the throttle! I had this problem about 4 months ago and I replaced the thermostat and it seemed to work until today. Do thermostats go bad this fast?? I am thinking it must be something else because it also looses water but I can't find any leaks and the oil looks ok...

Headgasket? Get a mechanic to test for combustion gas in your radiator coolant! It's a simple test!