Replaced The pressure regulator. put in a different fuel pump relay and she is still acting up. Hard to start and almost cutting out and then runs fine and the process cycles again. :x . Would the oxygen sensor make the vehicle cut out? Any info is greatly appreciated. Thanks you for your help.

Hi Folks, I have a 1990 740 gl with the regina fuel inj system. The vehicle intermittently stalls out and starts very hard. Codes are 321, 221 121. I have replaced the sensor in the tube from the airbox, I replaced the MAF by the brake booster, cleaned the throttle housing, put in a new fuel filter,...