I have a D750 camera body and a Nikon 24-70MM F/2.8G ED FX lens. Intermittently, when attempting to autofocus from 70MM, the lens will freeze and vibrate like it is trying to adjust but can't. The lens has been in repair three times but the repair facility is unable to issolate down to a fault or even replicate the problem. Has anyone else experienced this and if so, how di you resolve it?