Transmission shifting oddly on 1997 Oldsmobile Regency

I'm not sure if it's slipping or just plain not shifting properly. Started about a month ago at about 45-55mph. It would jump back and forth when trying to upshift, if I tried to gain speed it evenually shifts up ok and runs fine. Gotten more pronounced the past week or so. From about 35-50 if I'm on a slight grade then it does it. If not it shifts fine and it's fine as slower speeds even climbing a steep street. I'm really hoping this is not serious. I put 1/2 qt of fluid in it because it was low but did not help. Could it be a faulty speed sensor? Anyone who checks this out and responds I really appreciate it. I forgot to add, it appears if I shift to D3 it shifts ok if that helps.

Asked by for the 1997 Oldsmobile Regency
Possibly you also have a Check Engine Light or Service Engine Soon Light on yes?
I say this because it sounds like the Torque Converter Clutch of your transmission is wearing out, which is some what common on GM.

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