P0780 Saab - Shift malfunction

Question

Drives fine for an hour. Then the skid indicator comes on and transmission slips. Then the engine light and the sprocket with the exclamation point comes on. Then no longer downshifts. Need to manually drop it into first gear, which it really clunks into. When you shift back up to Drive, clunks into gear there. Next day, all warning indicators fine. Car drives fine for an hour.