Three possibilities basically: Either the valve body is dirty and some passages are clogged OR a valve is sticking in the valve body OR the K2 clutch pack seal is leaking/damaged.
Take out about 10 oz of transmission fluid and pour in a bottle of Lubegard 60902 transmission fluid protectant (red bottle). Give it about 150 miles and see if it goes away. If not then you may have to open up the transmission for repair.