1997 pontiac grand prix starts fine first time runs fine shut off starts up and runs rought

have a 97 pontiac grand prix 3.8 vin k car starts fine ses light on but runs fine shut it off and restart and runs rought ses light flashes i used a snap on scanner i get a po300 and ap1200 code injecter control circit if i let the car sit 6-7 hours it starts fine again im stumped on where to start i have no money and need some help in the right dir so i dont waist any cash
car has power and drives sweet up until a week ago