We've put up with this for over a year. We've noticed it's more likely to occur after 30 minutes and less than 2 hours of having been run. When it happens we just have to sit it out. Once we called out the AA and after a diag check he suggested a camshaft sensor might be at fault. He also indictaed after a time it usually resets itself (which is what seems to happen). He also suggested turning the engine over for more than 30 seconds might do the trick but it didn't work for us. We may have to bite the bullet and replace this sensor. If it works out we'll let you know which one and how much it cost. In the meantime, maybe a half-hour chill-out is not such a bad thing? Don't answer that, it's just wishful thinking!