Loose vacuum line and transmission not shifting properly on 1997 Oldsmobile Regency

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Transmission shifts funny from 35-55 if under a slight load. Shifts fine if not under a load and seems to shift ok too if I'm rapidly gaining speed opposed to gradual speed gain.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. It is fine at 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 modulator? No engine light on or other lights. Seems not to happe

I did try putting a pencil in a loose vacuum line but the car shifted even worse then throughout the whole shifting range.
Thanks Very Much
(1) Answer
If this transmission has a vacuum modulator, then make sure the vacuum supply to it is good. Fix any vacuum lines by replacing them, don't disconnect or plug any of them. Hope that helps.