Steering fault on Ford Fiesta 1.25 Zetec 2009?

my fiesta when you are driving along will pull to the right for a split second then drive ok. This has been reported to our local ford dealer. They have rotated the tyres and checked the brakes. They wont repair anything under warranty unless they have the car behave like that while they are testing it. Any ideas what this could be. Was wandering if it was an electronic power steering system (fly by wire) that has a fault.