2.0D Intermittant lacks power

[ChazzD]

Could do with some help. I've a '06 2.0D and have started experiencing a problem. It started on one of our really hot days recently. Was in traffic for around 12 miles doing a constantish 40mph. When I got on to a clear road tried to accelerate to 60-70 and the car is very sluggish. Would get there but took ages and just lacked acceleration. Took to garage and they found a code that pointed to the cylinder head sensor so this was changed. Fine until yesterday. Fault back now same as before. It will start from cold in the mornings fine and be great on drive to work (15 miles). If left at work in the sun (no shade in car park) will start up fine and will run fine for around a mile or two and then fault back. There are no warning lights up. But when I stop when it is failing the radiator fans are on full for around 5 - 10 minutes afterwards. Any clues any one ?

Thanks
Charlie