2004 Vue tranny shifts up & down alot

While driving my 2004 Vue the tranny shifts up then down repeatedly while maintaining a steady speed. It's like it can't decide what gear to be in or can't hold the gear that it is in. Suggestions anyone? ATF drain and fill? If so what fluid would you recommend ?
Also this vehicle hasn't been driven much because shortly after purchasing it used it made a horrific noise that sounded like a metal vibration and made the whole car shimmy and sounded like the engine was going to fall out! It has been to the mechanic 3 times and will not make the noise when they are trying to test it even if I'm driving it for them. They have been unable to figure it out as it happens sporadically there seems to be no Rhyme or Reason as to a certain speed or when it does it. Anyone have a similar problem or any ideas?