Engine trouble on an Espace 2.2tdci?

When driving on motorways,80+, no problem, then all of a sudden it just slows down to 70 or just above, then as your going up a hill, it starts to go down into the 60's, so you have to change down a gear, but still no power, then after a while, 10 mins or more, it just comes back to life,,,any ideas why?