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Ford taurus transmission problem when shifting from first to second

I have a problem with my Ford Taurus model 1996 , I had several problem with my transmission and engine. I had o change both with a higher model engine ( 1999 model and transmission ) but after doing that , while driving , I have a transmission problem when shifting from first to second shift specially when I am trying to accelerate .The car seems to jump while shifting .
What should I do ? Is it a VSS problem or it might be something else . Thanks to help

When you swapped-in the newer engine and transmission, did you also install the ECM and the TCM for the new engine and transmission? If not, then the problem is probably the result of a communication problem between the 1996 ECM/TCM and the 1999 engine and transmission.

Yes I did change the ECM /TCM

The transmission has to know the vehicle speed to work correctly, so it could be a faulty vss. But I doubt it. run the service manual tests on that part. If it tests ok, I think you are probably looking at taking advantage of the warranty that came with the replacement transmission and getting another one installed.