Another thing I thought about is a sticking shutter although that would happen most, if not all of the time. After checking the mirror, as above, and all is well, very carefully lift the mirror from the bottom (careful, as there are actually two mirrors, but as they are loosely linked to each other, they will both lift). You can now see the outside of the shutter. It should be totally clean, no dust, oil or mould! You can also check the movement of the shutter while holding the mirror up, however it opens and closes so fast that you probably will not see anything useful unless it stuck while you were watching. It may be worth a try, but if it is clean, then it is unlikely to be the shutter.