Try another card and if it continues, get support from Nokia on repairing/replacing the handset.
I am having problems with my n95. i have a memory card in the format sd hc, and it is playing up. It will store files i.e music, pics, etc for an hour or so, and function perfectly , but then for no reason whatsoever it will not find the memory card. I can go to Tools>Memory> and the card will be there, telling me how much space is free and used on it, but in gallery or media player,nothing comes up, as if its not inserted. while the phone is like this if i take new vids or pics with the camera then they show up but nothing else. there is a load of music on there that i cant access. I have tried formatting, error checking and defragging on my laptop, repeatedly removed and put back in the card, but nothing shows up. I am fairly technologically minded, i know my kilobytes from my megabytes, and i would greatly appreciate any help