My Subaru (automatic) makes a loud clunking.

My subaru (automatic) makes a loud clunking sound and jerking motion (in the rear) when it shifts gears at 40 km an hour. This happens when I start the car only. I can then drive the car for the rest of day, stopping and starting at lights stop signs etc. And it will not happen again. Also, it is inconsistent in that it does not happen every day but on average I would say it happens 7 times out of 10. I have had the transmission oil replaced. This did not help.