OK. I have seen this problem on here before but mine is a little different.

At 55 mph, my 94 XJ Sport keeps surging by about 800 rpms like it is constantly shifting into overdrive and back.

I went after all the usual gremlins and replaced the throttle position sensor, idle air control valve, and map sensor. I also removed all the grounds, cleaned all the contact points with a dremmel, and reassembled them. I added two new grounds as well.....One to the radiator support below the hood latch and one to a mounting bolt on the ac compressor. I have also adjusted the throttle/transmission kick-down cable.

I am at a loss on this one. It only does it when trying to maintain 55 mph.

I do have 35 inch tires on it so I'm wondering if that has something to do with it.

 

AW4?
If so, try lifting the brake pedal with your left foot and see if the RPM's stay constant.

 

I have a problem with the mount for the brake light switch. I have to move the switch occasionally to turn the lights off after I park. Does this switch send a signal to the trans?

 

Ding ding ding !
The torque converter lockup signal goes through the normally closed contacts off the brake light switch.

 

Awesome. Thanks!

 

That did it.

Turns out the brake pedal arm was bent and the switch was only contacting the edge of it which was allowing it to slip past. Took out the switch, bent the arm back until it was centered over the hole, reinstalled the switch, and presto.
Just took it 10 miles down the interstate with no problem.

Thanks a bunch.

Now all I need to do is replace the timing chain, harmonic balancer, rear main seal, oil pan gasket, and that speed sensor gasket on the transfer case.

 

Having similar issues. Drives fine but just started to act weird today going in and out of converter lockup.. took a look at the brake switch and found it sitting loosely in the bracket. Might order a new switch since it doesnt seem to clip in securely. Looks like a poor design here.