Nikon 18-200 VR at 200mm strong back focus

Hi!

I have this lens for 2 years and a half. And last few months I am finging out that at full tele (200mm) and very close focus distance my focus is on the taken photo about 5cm behind the desired focus distance. Than I have to set on D300 to compensate back focus to -15 and it's OK.

But that think is time-consuming, so I am asking you if that is normal with this lens, or not. If not... my lens is going for a Nikon repair very soon. And what are your experiences...how much would it cost?

Thank you very much for your answers!
Regards!