I'm new to the forum, so hi.
I have a brand new Ford Focus Zetec 1.6 TDCi (115bhp) manufactured on 18th November 2011 and delivered to me on 12th December. It's done about 1700 miles already as its a company car. Ever since I had it it has done a kind of stutter (just a single stutter on each occasion) when in 6th gear doing 70 mph (+\- 4mph). It was particularly bad on its first journey which was a 3 hour one, I ended up driving at 60mph, then got slightly better over Christmas. This week I took it to Ford because it was very low on coolant (although might always have been), they topped it up and took it for a drive but couldn't get it to stutter. Since its been to them it's happening more again, about 3 times in a 20 minute drive.
It is only in 6th gear and doesn't do it on demand, but seems to be when I either ease off or accelerate slightly like on a motorway or dual carriageway.
For info, I have driven a lot of hire cars before getting the Focus for the next 3 years, so I'm fairly used to different cars.
Anyone have an idea what might be happening?