When driving Rpms move up and down when letting off gas pedal?

I have a 1991 honda accord when the car is driving and my foot is on the gas pedal the rpms are ok but if i let off to coast or slow a little the rpms will move up and down between 1500 and 2000 every time they go up the car feels like it boosts itself then slows then boosts it has a delay when doing this like 1 sec before the rpms go up or down
also the car does not do this when in park only driving?
so i need a main solution to this i was thinking mabey a vacuum leak or somthing with the throttle or the distributer cap help and suggestions would be much appreciated

Yes it does sound like a vacuum leak. It could also be a dirty throttle body or your Idle air control.