Intermittent focus problem with 200-400 lens

I have a 3 year old 200-400 VRI lens that I use on a D7000. I bought it new and have used it lightly and carefully. It is a fabulous lens. I have only one camera right now so I can't try it on other cameras.

Every now and then, during a shooting session ( mostly BIFs ) it simply stops focusing. Then, a few minutes later, it will start to focus again.

To correct this I have tried taking out the camera batteries, detaching the lens, cleaning the contacts with an eraser. I brought it by my local camera store and they just went thru the list of actions I just stated. And, of course, it was focusng while I was at the camera store so I couldn't try it on another body.

I will send it in to Nikon soon, but I have a trip to the puffins on Machias Island next week and this is the perfect lens for that. So Nikon will have to wait.
Any suggestions, observations will be greatly appreciated.
Thanks in advance,
Mike