I've read several articles that state that it is bad to delete pics in camera.
it's best to format in camera before each shoot, then do not delete in camera..the way it was explained to me...since each file can vary in size, let's say you delete a 3 meg file, there is then a 3 meg gap in the file allocation table, if you then shoot a 5 meg picture, 3 meg of info might be written to that gap, the other 2 meg after the last picture you took..so your file is split in different locations which can lead to corruption..
there is also the possibility that your card is just going bad do to age...cards have a limited life expectancy of so many insertions and uses..