spundunit13

I recently bought a 99 pathfinder and when I'm in over drive at 66mph its at 6000 rpms so i have to stay off the highway and keep low speed but when i am driving in overdrive and i let off the gas pedal the rpms go all the way down like it's on neutral and when i push the pedal back down it has to get the rpms all the way back up to 5000 our so rpms before it starts pulling again. Any suggestions on what my problem may be are greatly appreciated. Thanks in advance.