88 Regal Limited stalls when put in gear

[jeremyaster98]

Hi everyone. My brother bought a 88 Buick Regal limited a few months ago with only 49k miles. He had problems with the engine stalling, which he thought was the ECU, so he had it serviced and that seemed to fix the issue. He gave the car to me, and now it's starting to stall again whenever I put it in gear. Sometimes it'll jerk with difficulty accelerating for a few seconds before returning to normal, if I'm lucky. Also before this started, sometimes it would stall when turning on the AC. My other brother thinks it might be the transmission fluid since it's almost 40 years old and has never been changed. Can anyone share some advice and help me figure this out? Thanks!