I have been having a similar issue. Doing star trails set for 30 second exposure with 20 second gap in between exposures and set for 450 exposures. tried this twice and the camera stopped at 179 exposures the first time and 129 the second. I can't believe it's a write speed issue given the 20 second gap between exposures - set at JPEG medium. It is a Class 10 16GB card also. Given previous experience in burst mode, this pace of writing to the card should be no problem. Could it have something to do with the sensor given the continuous long exposures?