2003 Ford Ranger RPM Drop when ideling

Recently my check engine light came on. The diagnostic check at auto zone said it was one of the upstream O2 sensors. I drove it for another two weeks before I was able to work on it. In those two weeks it got to the point at a red light the RPMs would drop from a normal of 600 RPMs to 200 RPMs and it would cut off. I finally replaced all spark plugs and both upstream O2 sensors. That cleared the codes and it ran fine for a week but it started doing the same thing a few days ago. I was hoping someone might know what the problem could be. Thank you.