You are not unique. I had exactly the same problem with my D300s last September. To add insult to injury the fault emerged during a leg of an international Catamaran racing event in the beautiful natural harbour of Plymouth UK.
Like yourself, I tried everything I could think of and other things which fellow camera club members suggested. Finally decided that it was a return to Nikon job.
My camera was returned with a repair report stating "Printed circuit board repaired,replaced and adjusted". The camera has been working perfectly ever since