Fault Code P1299 Overheating On Throttle Back Off

2003 Ford Focus Zetec 1.6 UK

I have recently had the temperature warning light come on while driving on a motorway for about half an hour which was followed by the engine management light coming on and the car going in to limp home mode and firing on 2 cylinders to cool the engine. Fault code P1299.The car restarted after turning it off and drove home normally with no temp warning firing on all 4 cylinders.

I replaced the thermostat and the Cylinder head temperature sensor but no joy. Actually the new thermostat was slow at opening and caused more problems so I replaced the old one which when tested opened quicker.

The car now runs Ok, Temp goes up slowly on tick over to about 104deg C then the fans kick in and go off at 99deg C. Everything seems OK, you can feel the water circulate round the radiator when the thermostat opens but when I drive the car the temp stays about 104 deg C or so until I back off the throttle then the temp almost instantly goes up to 122deg C and the gauge goes straight into the red, temp warning light and engine management come on.

Once I start to use throttle again temp comes back to normal within about 30 seconds. The engine doesn'et actually seem to be overheating though.

I have tried screening the CHT temp sensor with tin foil to see if it was interference from the ignition leads and tried leads from a different car which didn't help. The coolant has been refilled by using a vacuum pumps to get rid of any airlocks which seemed to work well.

I have now run out of ideas and am starting to think along the lines of ECU problems.

Any ideas or previous similar experience would be gratefully received.