1995 Dodge Stealth not getting any real heat from heater co

Just got the car and the dealer flushed the heater core. The temperture of the air improved a little. I drove for over an hour and the gage was almost up to halfway when just like that it went almost down to "c". I dont have repair history for this car. Should I replace the thermostat and coolant temperture sensor? Is there there something else.