Thanks for your contributions. My SB800 is always properly seated in the hotshoe and locked. I use Alkaline batteries and a fresh set does not help when this problem starts. When I leave both camera and flash in off position for a while, the come up without issues but resume after a while.
I had seen a post somewhere that suggested cleaning the contacts. I did that but did that after I had had both camera and flash off for a while. The problem returned after a while, meaning that it was not contact problem. My big concerb is that in the waiting time, I'd have missed some good shots.
I wonder if anyone else has encountered this and how he got round it.