Question Torqueflite A904

Michel said
what happens is that in many situations on flat / straight roads in full throttle I relieve the throttle before engaging the third gear and instead of continuing in 2rd, it jumps to 3rd !!
I have little experience with automatic transmission and I was doubtful if it is normal, or adjustment is needed or there is a problem

I just told you.:D
You can adjust the throttle pressure all you want and it's still going to do that, just at different throttle openings and different road speeds.
It's not a manual trans, so if you want it to stay in second without upshifting then you have to keep it manual second until you want third.
Or maybe get a full manual valve body, as those will be in the gear you have selected at any speed or RPM until you manually change it yourself.

In decades of professional transmission building you would be shocked at how many times I have heard that complaint.
99% of them with older transmissions like 904, 727, C4, C6 and some of the others.