I think this issue may be due to using invalid readings from the temp sensor. The invalid readings seem to be caused by noise introduced by switching the relays on. So what happens is you are in heating mode with the current temperature less than the setpoint, so the Model-T switches on the relay. This causes the sensor to report an invalid value of 999.9 degrees. This is above the setpoint, so the Model-T switches off the relay. Then the sensor recovers and starts reading values below the setpoint and turns the relays back on.
The update I pushed last night adds additional checks for valid sensor readings and might help this issue. Can you please try it out and let me know if there is any change?