Thank you for your suggestions. I reformat the card in camera after transferring the files to permanent storage. The number of files on the card has no noticeable effect on the "lock-ups" I have been experiencing. My quality is set to lossless compressed RAW plus jpeg fine - probably the most processing time consuming setting. I will try running with only lossless compressed RAW as I always transfer my files directly to the computer, view them in ViewNX 2 and do most post processing in Capture NX 2. If I want a jpeg file, I create it in Capture NX 2. Also, as my main area of interest is landscape and this is where I've encountered most problems (probably by going too quickly through exposure bracketing at a couple of different aperture settings), I can afford to slow down and give the camera time to do its thing.