I have a 96 Bonneville, I got it from my father, and when I got it, had the CHECK ENGINE light on. Well I found out that the MAF sensor was bad, the MAP sensor was bad, and also had a P0753 code in the transmission. (shift solenoid A, electrical fault)
I replaced all of the sensors, tuned it up, had the oil,coolant, and transmission fluid changed, thinking that would help it. But, no it did not.
I took it to a trasmission shop and they said that the shift solenoid was working and that something else could be causing that fault to be thrown..... Well I can also tell you that when it starts out cold, it shifts up and down up and down and then catches. Also, the passing gear does not kick it, when you mash the accelerator.
Could there be something simple to this....... Does the transmission have any vacume lines, or relays or modulators.....(not sure about where the modulator may be, if it has one).
And where can I find out what kind of transmission is in the car......
If I do need to change the shift solenoid, Is it something a partialy mechanical guy can do.
Sorry for the long post.
Hope someone can help out.

I do believe there is a vacuum shift control, but I really can't remember where it is. Check all your vacuum lines for a leak.

When you lift the hood, look down to the right at the trans. The vacuum modulator is right on the front and fairly easy to get at and see.

The vacuum lines are prone to leaking and having issues as they have aged. Definately check them out and replace most of the fittings etc to ensure this isnt' the issue.