Re: Possibly transmission or electronics
The transmission is a 4-speed with TCC - Torque Converter Clutch - which is supposed to engage at light to moderate road loads above ~43 mph.
3rd gear is the 1.00:1 gear; 4th is a 0.68:1 gear - OD. The TCC removes the last 300 or so rpm of slip between the converter stator and rotor - it's a fuel savings feature.
Drive the car normally in D - you should feel it start in 1, then 3 shifts up to 4th above 40 or so (load/power dependent).
There are 3 shift solenoids - 1-2, 2-3, and TCC engagement. The 3>4 shift is speed/load/power/fluid pressure dependent.
Has the transmission ever had a GM dealership fluid exchange service performed ? (Not to say that will help anything - just a maintenance feeler)
A diagram of the transmission sensors and switches in my albums.
EDIT: PM question - ???Albums ??? Click a member's username and look at their profile. Albums and such in the left column. If an album is kept current, it (they) can be accessed through the Community tab up ^^^ there.