Hello, I apologize for the long post, but I would appreciate any help..

My grandmother's 97 Deville has had some transmission issues.
Starting from a standstill, once I accelerate, it would sputter and bounce (?), best way to describe it.. before driving and would accelerate slowly. She rarely drives her and when she does, only goes 5 miles and doesn't pass 45 mph, been like that for about 12 years..
i drove it and have been trying to diagnose it. When flipping through the gear selector, it seems like 1 is my issue. It's like it doesn't even try, it just stays in 2nd or 3rd. The following day, I drove it around on the highway and would just floor it and all that to clear anything up and after about an hour, it drove like I remember it did 6 years ago - from a stop and accelerating, it would run through all the gears. The following day, back to square 1. It Recently had a transmission fluid flush.
After it sits overnight, and start it the next day, start accelerating slowly, let off the gas and it will like drop down and the traction control and abs light will pop on. That's when I notice it not really starting in 1st anymore.

Other notes/issues: Will randomly cut off. For example, i was in the drive thru and inched up a car length and it cut off, turned over right away. Not sure if it's a cause, but the coolant temp was ~235. The spedometer, gas guage, trip, odometer stopped working

thank you for reading!