Dirt
There's very probably a dirty connection in the plug - cut it away with a few inches of cable and fit a new one.
If the wires have become slightly dirty from sparking say - a loose connection - soot will build up causing a high resistance contact - if your machine is 2kW for instance - and the resistance of the connector in the plug perhaps just 2 Ohms then the wattage consumed by the poor connection is the current squared multiplied by the resistance - over 150 Watts!
Look to the connections inside the plug.
Good luck...
M Faraday, October 2014
