'98 Windstar Fluctuating Heat

My wife has a '98 winstar we just bought in the fall. When it got cold the temperature would not go up on the temp. guage. I took it a mechanic and he replaced a bad thermostat & drained and flushed the cooling system. Now it will eventually heat up (if it's not to cold out), but when it is heated up a soon as it exceeds 2000 rpm's you can watch the temp guage go down and the air from the vents cool down. I took it back to the mechanic who worked on it originally and he could find nothing wrong with it, he let it run for an hour and said it heated up fine (he used a heat gun on the engine). I took it to another shop thsi weekend they couldn't find anythin either, they let it run for a while and they said they drove it extensively.

If anyone has an idea about this please help me!!!!!!!!!!!!!! My wife is tired of driving in a cold vehicle (and so are the kids).

:banghead:

I would replace the thermostat again, probably a bad one from the stock room.