Check the oil level in the transmission first, low oil can cause this on some vehicles. Also check the condition of the oil, if very dark with a burnt kind of a smell, strongly reccomend to change oil, as it will have lost it's lubricating qualities. If this does not completely solve the problem, there is an additive you can add (only once) to extend the life of the transmission (wynns recommended) but if it still plays up, it will need professional repair/adjustment

Check your transmission fluid level. If you haven't changed the fluid recently, change it now to see if there is some blockage in the veins of the transmission.