The buffer size will not change with a faster card, the buffer is a fixed size so will always be at 5 for RAW. Using a faster card allows the buffer to be cleared quicker, so that as you continue shooting the buffer is emptied quicker to the card. What other settings do you have enabled? Adding original decision data will slow down image capture, as will other in-camera image manipulation settings. Try turning them all off and see if it improves. You would be getting quite a few shots off if using jpeg before the camera stalls.