98 Lumina Problem (tranny?)

I got an 98 Lumina with a 3.1L motor with 170000kms. When in overdrive the tranny will create a unusual shift between gears, seems like it slips for a
couple seconds. this problem will occur at any random point but usually at 15km/h or around 70km/h. also seems to happen after ridding for 15mins or so, but will occur when machine is cold. What could be wrong, over drive broke? easy to fix?. Oh, also it will never happen driving in regular D, or 1,2,3, only in overdrive.
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Needs to be scanned on a bi directional scanner to see whats happening internally. Any good trans shop can handle this.

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