Sounds like a vacuum leak, but since there is no serge in the idle in drive or park I don't know. But then again I'm not familiar with automatic transmissions so don't take my word for it. If you wanna check for vacuum leaks simply follow each line with your hands and see if you feel any sucking. If you do, that could be your problem. Remember to check the PCV valve, with age they crack and alot of people miss that one.