Mondeo diesel hunts at constant revs?

Hi there, I have a 53 plate TDCI Mondeo and it has started to hunt when at a constant speed. No problems starting or accellerating, just when sitting at 70mph for example. It feels like there is a slight fuel blockage but works fine when I put my foot down. This has happened after the car being left idle for 2 weeks. Any ideas?

Scubatt
September 2012