Q: I keep having acceleration problems, the mechanic keeps saying catalytic converter 2006 Pontiac Torrent

I have a 2006 pontiac torrent and I keep having acceleration problems the mechanic keeps saying catalytic converter but I've replaced it 4 times with the same problem returning the check engine light codes says it's running lean with catalytic converter codes as well it will be ok for a few days then start doing it again...when I'm driving it will go into a coast not losing power I'll push on gas car will backfire then eventually will accelerate again but initially with a big surge of gas causing rpm to go 5 then be normal a few more miles and will repeat problem and the problem is getting worse. The dealer sent it to Buick to have a full hour diagnoses and the result was the same a bad catalytic .... As an added note the dealer just put in a remanufactured engine in which the light was on after picking up the car after that was done

My car has 119011 miles.
My car has an automatic transmission.