It shouldn't have anything to do with the ecu as the thermostat is mechanical. Make sure you have a good thermostat in the engine. Even if it's new, they have been known to be bad right out of the box. Go with a good quality name brand like Stant. Otherwise OEM is always your best bet.

Secondly, make sure you don't have your interior blower going at full blast. The heater core is like another little radiator. If you have the blower going at full blast, the engine will stay cooler for a longer period of time.

Good luck and I hope this helps.