Citreon Picasso Diesel, 2001, has displayed message,?

My Citreon Picasso Diesel, year 2001, has started to display message - Stop, engine overheating. It has shown this today and last Saturday, as soon as i turn off the engine for 5 minutes then restart everything seems fine. This problem happened approx 3 years ago, I had the car checked but no problem was found.

Mark D
August 2009