DR3WS

I have a 97 cav 2.4. Last year my exhaust manifold broke and fell off, the replacement was over $400, so I opted to get a header for it instead. And I also have an aftermarket muffler. Since the header installation I keep getting check engine light. When I start my car it will run smooth for about 4-5 sec, then it spits and sputters nearly dying for another 5-10 sec, after it does its thing it runs great. Codes imply its definately exhaust issues. I'm thinking it might be low back pressure, and that putting in an o2 simulator might help with codes and rough start up, any ideas?