Based on my experience with my 1998 Mystique V6, change the waterpump and its drive belt ASAP. Don't drive the car any distance before fixing this. Try to get a metal impeller on the new pump, or white plastic. Check for broken pieces of the old black plastic pump impeller. If the thermostat was just changed the t-stat is probably OK. The temperature gauge sender should be replaced, unless it is just simply unplugged. My sender was shorted out, which pegs the needle right on full hot. The sender is the brass hex head item to the right of the blue cap in the picture below, (Not the circled Engine Coolant Sensor) The other thing to the left of the blue cap is the waterpump and belt If the radiator cooling fans operate "vigorously" with no odd sounds when the a/c is switched on, they are probably OK, and should trigger when the engine temp is at M on the scale.
