Have you noticed any pattern with the stuttering problem? Does it only happen when you're in a certain area? The type of scenery and objects in your view can have a big impact on frame rates. For example, are your FPS high when looking out over water but the stuttering starts if a city comes into view? Is it in every plane? Or does it happen when you're in straight and level flight, not touching anything?
If there are any patterns you can find, it would be helpful to know to try to reproduce or test this.