I once had this problem and found that the lower mounting bracket was slightly bent. This caused the starter to set at a slight angle and made the bendix/starter gear to engage the flywheel too tightly. The gear would not release from the flywheel until the motor had made a few revolutions. You can check this by engaging the bendix gear manually, with a prybar, while turning the flywheel by hand to mate it. Once engaged, a slight turn of the flywheel should allow the gear to return to its resting position. Good luck.
