Jeep Wrangler Trans problem.?

Ok i know my friend will most likely have to have a complete rebuild but its worth a try.

He has a Jeep Wrangler i believe a 97 4 cyl automatic and he's having major trans problems.

when he's shifting into 3rd gear it will slip and you have to let completely off the gas, have the engine almost idling and it will go into 3rd. also if you press too hard on the gas it will slip, and you will have to let it go to almost idling again just to get it to get back into gear.

he just bought it, and the guy said that "it was sitting for about 6 months" we've done all the fluids flushed and changed (oil, transmission etc. etc.), all the filters changed out, thermostat has been replaced, and still it does this.

When i say it's slipping, i mean it completely goes out of gear like you would go into neutral you can rev the engine all you want, but its not going to gain any kind of speed until you bring it down and start again.