Galaxy Auto 1.9 Diesel-Loss of Speed?

Whilst driving once i hit 50mph there is no more power even when my foot is to the floor, this doesnt happen all the time, & once i turn the engine off & back on again its fine, it doesnt happen every time either but its quite scary when your on a motorway.
Look forward to any suggestions of what it could be,
Thanks

Alan
August 2010