85 300CD intermitent starting cylce

My 1985 300cd started acting up today. Car runs great... however today the Car did not start. I insert the key, turn the key to show glow plug lite on, wait for light to go out and then turn key. the key does not turn ( i mean it will not rotate clockwise anymore). Then i will rotate key back to off, pull key out, cycle the transmission from park to low to neutral several times. Then insert key and try again and sometimes it starts and sometimes it does not. Basically i Keep doing this procedure till it starts. After i start the car i drove the car 40 miles with no problems. Reached my destination, turned the car off and tried to restart and had the same problem. Followed the same procedure as above and Finally got the car started and then drove home no problems... I am open to any suggestions

Thanks in advance....