Help!!! Power Loss

[Ttime89]

Hi any help would be much appriciated, i own a ford focus 1.8 tdci 2007 the problem started about 3 weeks ago when after the car running fine for about an hour on a fast moving A road as i slowed down to approach a round about i dropped into 3rd as a put my foot down i lost 95% of power thinking i may be in to high of a gear i droped into 2nd then 1st but the same problem i was going 5mph max with my foot flat on the accelarator the revs were slowly going up but usualy having the car in 1st with full throttle should sound like the engine is about to blow surely??? I turned off the ignition and restarted the car and it was fine, this has since happend twice since then exact same scinario approching a roundabout at speed breaking dropping down gears then boom no power and will return to normal after turning key off and on, thanks for any help