I have a 97 150 with 234,000 miles and completely stock. A few weeks ago I noticed the rpms going from 2000 to 2200 and back again within a second or so. I was driving on the freeway at about 65 with the cruise on. Turned off the cruise and still happened. Got off the freeway and drove in town at about 45-50 and it was still doing it.
Got home and replaced fluid, including torque converter, and filter. It took about 12 quarts of mercon v. Drove and it was gone. About a week ago it started again, but now I've found it only happens in OD. I have a 4.6 with a 4r70w. Any ideas?

 

It may be your torque converter locking and unlocking or OD shifting in and out?