Manifold/header/muffler/tail Pipe problems of the 2005 Hyundai Elantra

On 10/24/09 I was driving my car for approximately an hour and a half when I noticed a slight jerking feeling. It repeated once or twice. Later when driving home, after driving for about a half an hour it started up again. Then about an hour later it stalled at a red light, I hardly made it home. I took it to the mechanic and it wasn't displaying the problem. It started up the next day and the check engine light came on. It pulled a po 420 code. I change the O2 sensor, didn't fix it check engine light back on same code. I changed my mid cat converter didn't fix it, light back on same code. . . My mechanic thinks we should change upper cat converter, I call the dealer. They say the upper cat is connected to the manifold which means I'm still cover under the 100k warranty. Mind you I had my manifold changed at 72k because the check engine light had gone on 18months earlier (I hadn't thought anything of it because it was under federal emissions warranty. ) now I'm at 99k and need a new manifold???? so the dealer replaces the manifold, still not fixed. Break down on side of the road. Dealer says it's my problem, they fixed it!!! back to my mechanic po 420 again. He thinks the old upper cat blew out the new mid cat before we replaced it. It's under warranty so I wait a week for the part to come in. . . You guessed it still not fixed! I'm bringing it back to my mechanic tomorrow and calling the owner of the dealership. It's been 7 weeks, $900 and my car is still broken.