stonewolf

I have spent many years learning the ins and outs of my Cherokee and this forum has always held the answer for me. Except now. I can't find any post with similar issues as I have encountered lately.

Situation: The vehicle RPMs drop abruptly then shoot back up over and over when under load (ie. quick accelleration, towing, or anything over 65mph.) I am also hearing a whining noise deep in the back of the engine or front of the transmission.

Vehicle: 1987 Jeep XJ 4.0 Auto AW4 4X4

Backstory: Tow months ago I towed several trailer loads of about 1000lbs using the power setting in overdrive from and elevation of 1200 to 7200ft. From Phoenix to Prescott, AZ for those who know the area. On the forth trip I noticed a slight hesitation with an empty trailer. A few miles later it would jump from 2100 RPM to 1500 and back up quickly over and over. So I stopped the vehicle and had AAA tow it to AMCO Transmissions. They said is was not the transmission and that some wires, spark plugs, and the TPI needed replaced. So they replaced them. I picked up the Jeep several days later and started to drive it home without the trailer. 10 minutes down the road it started again. If I switched it to neutral while still moving at 45mph the RPMs and gave it a little gas it would still drop and jump slightly. When I pulled over and stopped, I switched to neutral and could hold a perfect RPM for as long as I wanted. The engine ran flawlessly. I had the Jeep towed home and replaced the fuel filter, TCU, transmission filter, and fluid. I checked the shift solenoids and they were all between 13 and 14 ohms. I also checked the transfer case and it was a pint low so I drained and replaced the fluid with ATF4. I also check the diff cases and the gear oil was good. The Jeep is still acting up and not sure what else to try. I did add a header system over 8 months ago and never hooked the O2 Sensor up because I need to have a hole added. But it was running fine up until last month. The whining also got louder and noticeable while I was testing the Jeep at my house. It doesn't matter whether I am in the Power or Comfort setting or in any specific gear. If I baby it I can get it to 65mph before she acts up. Otherwise if I stomp it or go up hill she will act up at any speed. It literally feels like something is physically grabbing the flywheel for a split second over and over when under any load.

Not sure what else it could be besides the torque converter or transmission. But typically if a transmission is slipping the RPMs go up not down. Any other ideas?