Engine stalls after warming up wont start until cools down. Gives code for O2 sensor

Vacuum? Catalytic conv? Bad O2 sensor? '92 bmw 525i : It has been running ok but low fuel mileage and exhaust leak. Muffler shop tightened exhaust manifold and fixed leak. Ran for 1 day then extremely low rough idle and surging as driving and fault code for O2 sensor. I replaced leaking PCV elbow and grommet at brake booster and a cracked air intake tube. Car started and idled perfect but stalls after 5 -10 minutes and wont restart which I assume is when engine is up to or close to operating temp. Before replacement of 2 previous mentioned parts, I could keep it running by constant rev or when it surged I could stomp it a couple times and it would regain power. Help please this is an elderly friend's vehicle and she has no money so she is counting on me