If thermostat and timing and water is circulating right what would make a tbi 350 get hot at first and then whenit opens work fine after that unless its cold?

If thermostat and timing and water is circulating right what would make a tbi 350 get hot at first and then whenit opens work fine after that unless its cold?
either faulty thermostat not opening soon enuff, or wrong temperature thermostat still not opening soon enuff