Rough Idle

1995 900S 2.3 (non turbo) automatic Here's the issue. My car idles fine when it is in park but as soon as I put it into drive or reverse it will idle rough for 3 to 5 seconds, it kind of oscillates up and down 100 to 200 rpms and then it smoothes out just fine. On the road it seems to run fine but then even if I go from drive into reverse it will again idle rough for several seconds. Any ideas on what the issue might be.

Thanks, C