Take off the lens and fire the shutter, look at the mirror's action. Is it rising all the way to the top? Before the shutter travels? Open the back and, with the lens removed fire the shutter observing the action at various speeds. Point the camera at a well lit blank white wall for these tests. This should tell you if it is the mirror not getting out of the way soon enough or, if it is the first shutter segment dragging, allowing the second segment to catch up and end the exposure.