4.0 power loss in 5th gear

I know this has been mentioned on here before but I can't find anything with the same symptoms I am having.



I have 1992 with a I6 4.0 5 speed Jeep runs good until I get on the highway hit 5th gear and then a hill. I go from 70 to 55 in a few second depending on the grade of the hill:ballchain: and then I have to down shift to four to get back up to speed.



This slightly puzzles me from what I have read about other peoples problems. I have all kinds of pick up until highway speeds. I never had this problem in my Cherokee with the same engine and 5 speed.



I put new plugs(Champion) and wires, New fuel filter. The basic things I could think of first. Now my question is could it be the Cat or maybe the Fuel pump?



Here is the other thing I have noticed and this is what makes it more wierd. In the mornings when it is still cool and I am on the interstate the jeep has no problem keeping up speed, but in the evening when it is warm or hot like it is today it struggles. I am just at a loss:confused: and wondering if anyone has any ideas.

Thanks for your help