I have a Nikon 70-200mm VRI lens and recently it seems to want to do erratic things when the VR is being used. Not every time yet, but sometimes, the VR will just throw the glass up and down sorta violently. I can usually just turn the camera on and off and it'll go back to normal soon and it works like normal for a while again. It's was on normal mode, not active, for the last time it happened. I am just wondering if that's a sign of imminent failure?