Why does my 96 jeep cherokee go into neutral when it tries to shift from 3rd gear to 4th?

when my 96 jeep cherokee shifts from 3rd to 4th it goes into neutral most of the time but other times it shifts perfect but when the gets a little hot it does it everytime. sometimes i can shift into neutral and back to drive it will kick in overdrive. now when in overdrive i can kick it down so to make it go into 4th like when im ping it will work perfect. it has 125000 miles but i have a 92 cherokee with 337000 miles and no prob at all with (knock on wood). i have changed the fluid and filter and checked what i could. i can rebuild that jeep from top to bottom but an automatic transmission is a mystery.