I suggest looking very closely at the wiring for the heater connection to the hotend electronics. Inspect the wire going into the connector to make sure the pins are fully seated and/or the wire is not starting to break.
Also trace the wiring from the connector on the hotend electronics back to the controller board; inspect the wiring into the connector for breaking wires and the pins are fully seated. Inspect the insulation on the wires running back to the controller for cuts or breaks in the insulation.
Another problem point is the wires going into the heater tube; look for shorted wires and or a breaking connection.