BMW,I just got my 99 BMW 528i back had a transmission put in was running fine but out of nowhere when I put it in gear it’s stallin

Just got my car back. Had a transmission put it. Was running fine until yesterday when I put it in gear it's stalling I put in octane booster because it was sitting for like 4 months so figured bad gas runs when it's idle but stalls when in gear

My car has 217000 miles.
My car has an automatic transmission.