2005 Ford Taurus bucks at 18-20 mph

Codes indicate possible bad catalytic converter, but I already replaced it. Codes went off for a while after replacing, then came back on. Not sure if pertinent, but I know a bad cat can cause a bucking. So maybe something else is making the cat go bad or has similar codes? Misfire/ lean / etc

Anyway...

when coming out of a stop, car bucks at around 18- 20 mph. I can sometimes prevent it from happening by either accelerating slowly past that mark, or letting off the gas right before it then accelerating again.

If I have a long drive (2 hours+), sometimes it doesn't do it at all or as much after that for the rest of the day.

Doesn't usually buck when coming out of a stop and making a wide turn (like an intersection).

Doesn't usually do it if I slow down to go under 20, then accelerate again without actually stopping.

Thank you in advance for any help .