it is a very common problem and is norm due to the green relay over heating, well it over heats due to the fan plug on the thermo switch on the thermostat housing being fulaty, people tend to pull them off with the wires and break them inside or they burn out, so i say look at the plug then check the relay and clean the pins on the relay.
may be worth also spraying wd40 in the thermo plug as well as the plug on the fan.

oh and check its not rubbing on anything making spining the fan hard and causing it to over load the wiring