Nikon lens not focusing at high end and at far distance?

I have a nikon d40 with a 18-200 mm lens.latley if i try to shoot something like an airplane it will focus but won't be clear and it won't take the photo unless i swtch to manual or move the focus ring all the way past infinity… It use to take them fine… But its only at far distance. If i go to take a picture of a flower on the ground zoomed all the way out just like the airplane it takes the photo of the flower…

i have insurrance on both the camera and the lens but i don't know if its the lens that needs replaced or the camera?

I see no reason to say that the lens and camera are malfunctioning. They are doing what their design permits them to do. For one your using auto focus, that alone counts for 99% of your problems, the deal is Autofocus will not go the extremes that the lens is capable of. So stop that.

You should not be letting the camera make decisions for you. Think about it, do you think the camera is smarter than you are? Its not true, no matter who you are. Cameras are dumb, you are smart. Its amazing how many people I come across that think the camera is smarter, but its simply not true.

take it off autofocus and do it!

I agree that manual focus is going to be more reliable, but I just grabbed this shot today using autofocus. Be sure that you get the plane dead center in your focus spot and track the plane as well as you can while the camera is trying to focus.

To test your lens, try it on something else that is at a great distance that is not as hard to capture as an airplane. Look in your manual (page 27) and you will see that things like airplanes in the sky are not good subjects for autofocus. "There's little or no contrast between the subject and the background."

Autofocus is quicker and easier and is not something that is avoided. You just need to know the limit of it.
Now the lens and camera combo you talk about isn't the best for autofocusing, but manual focusing on a AF lens is even worse (the focus ring has far too little travel).

So you just need to understand that you need a strong point to focus, so if you're shooting a blue aeroplane against a blue sky then the camera will struggle.

However, what you talk about focusing past infinity makes me wonder, have you dropped your lens to put something out of alignment (even the ring may have slipped), because although the Infinity hard stop is past optical infinity, you should never be able to achieve focus past Infinity.