98 Buick Tranny

Okay Guys and Gals here is the story.

My wife drives a 98 Century 3.1 . Normally on the highway at 70-75 mph
the car does the surging thing. It feels like the TCC going in and out. No codes and no problems except it is annoying. Here is the interesting part. In the last week the temp has been below 60 degrees. This issue with the car has gone away. The other day it was warm so I drove it to work and it was surging. On the way home it was 40 degrees and it went away again. I would like to make it go away permantly if possible.
I am thinking a inline tranny cooler, but could the issue could be coming from the PCM under the hood?

Any Ideas?

public, I hope you find a solution mine is doing about the same thing. Mine feels like a miss or surge under load. Good luck with it.

Yeah, it is not a miss. and under load it does not do this. mine is a steady speed. It feels like driving on a windy day into the wind. Just barely noticeable as the TCC unlocks. If I drive at a higher or lower rate of speed it goes away. It has been this way since I have owned this car.(it was cold weather when I bought it and so I did not notice it then).