A flashing display indicates that the heating element is disengaged and the kettle is not heating (i.e. the kettle is "idling"). This happens when the hold switch is turned off, the kettle reaches the selected target temperature and, either,
1. The kettle sits for 5 minutes
2. The kettle is removed from the base.
If you'd like the kettle to return to the target temperature when you place it back on the base or maintain the target temperature for more than 5 minutes, turn the hold switch on. Otherwise, when the screen flashes, the target temperature must be re-selected while the kettle is not seated on the base.
With hold function enabled, the Stagg EKG maintains the target temperature for 60 minutes. When the kettle is removed and placed back on the base, it will register a drop in temperature and begin to re-heat to the the target temperature. After 60 minutes, the heating element disengages and the kettle will idle for 15 minutes before turning off (i.e. the screen shuts off).
Note: If you notice your kettle doesn't hold the exact temperature and oscillates a few degrees, don't worry. It is likely operating as intended. Rather than having the heating element engaged constantly in hold mode, it pulses on/off and targets the temperature you selected. When it senses the temperature dropping, the heating element kicks in. When the temperature exceeds the target temperature, the heating element shuts off.
So, lets say you set it to 200°F and begin pouring once the kettle reaches temperature. When you place it back on the base and the kettle recognizes the water is now 198°F, the heating element engages to bring it back up to 202°F. When the kettle reaches 202°F, the heating element disengages, but the temperature can continue to creep (up to 203-205°F in this instance) before ultimately dropping to the selected target temperature.