Renault Modus engine loses power

Engine keeps loses power when driving around 60-70mph, like it has run out of petrol. No warning lights come on and evenually it regains power. I've taken it back to the dealer as it's still under warranty, but they are reluctant to do anything with it because the 'cumputer' isn't registering any problems. Can anyone give me any ideas as to what the problem might be? Thanks.