On my little 1.4 this happened, done all the rebleeding, replaced relays, replaced fans, removed relays and bridged the terminals for it to come on with ignition, replaced temp sensor.
Then decided to take the thermostat off (should of realy thought about this sooner) and it fell apart in bits, replaced it with a new one and over the past 2 years it's been back to normal until 2 weeks ago it happened again at my local tip and the car decided to drop it coolant all over the floor, filled up rebled and took it home yet to see if it's the same thing though as not had much time.
Don't know if that helps, but just an experience i had.
Then decided to take the thermostat off (should of realy thought about this sooner) and it fell apart in bits, replaced it with a new one and over the past 2 years it's been back to normal until 2 weeks ago it happened again at my local tip and the car decided to drop it coolant all over the floor, filled up rebled and took it home yet to see if it's the same thing though as not had much time.
Don't know if that helps, but just an experience i had.