1997 Jeep Cherokee Please help.

Engine Performance problem
1997 Jeep Cherokee 6 cyl Four Wheel Drive Automatic 268 k miles

the car description is as that, the problem is when the engine is cold the car grear is in drive but it drive with the rmp at 5000 to 6000 rpm but the car goes only 40 km per hour. Eventually the engine get hotter the rmp is same or higher the car loses power and slow down slower to 2 or 3 km per hour. I change the trans fluid and filter. Transfer case and differential front and back also got fluid change. All the fluid is accordding to the manufacture. Please help.