2000 Pontiac Grand Prix GTP 73,000 miles

My car has been misfiring randomly after start up for about 2 weeks. It cleared up after driving for about 30 seconds and would return again the second or third time I started it up after that. It started last year at this time and was showing fuel injector #3 malfunction. A dose of fuel injector cleaner in the gas tank fixed until now. This time I had it hooked up to a scan tool and it showed that the fuel system was running in an open state even though it was at 200 degrees F. It showed no reading on the 2nd O2 sensor and randomly showed a malfunction for the same #3 injector and a misfire. I gave it another dose of fuel system cleaner and changed the spark plugs. It seemed to run smoother with the new spark plugs but the problem continued. It has run through 3/4 of a tank of fuel with fuel injuector cleaner and seems to be getting worse. It rarely stumbles during idle right after start up, but it now does it while I'm driving. If I give it a lot of gas on the highway, it will clear up, but through town it will just keep doing it until I open it up on the highway again. Please help, I already have a Jeep Cherokee that eats money, I wanted this to be my dependable transportation and not have to sink money into it all the time. Thank you for your time.