One thing I do when shooting slabs is back the lighting angle off somewhat, trying to minimize the glare from the slab - even though it may not be affecting the coin itself, it may become something the camera wants to focus on.
Does your camera have the ability to specify focus points? What model Coolpix is it?
The best thing about a bicycle is that it uses no gasoline, therefore the chance of fiery death is greatly reduced.
First Catman, then Gary Burke and now Bigg Fredd - there's one heck of a coin club in Heaven.Our members sell on eBay!