P0430, then P0153

I just bought an '03 GT a few weeks ago, and last week I had a CEL with a P0430 code. The dealer replaced two sensors ( I know there's 4), most likely the two up before the cats. Five days later, I get another CEL with a P0153 code. I know what the code means, but what should be done to fix it? Replace the other two sensors, new cats? The car only has 30K on it. The exhaust system sounds way to mean to be factory but I haven't had the time to get underneath and see what's actually there. The car does have a CAI. Any ideas? I want this to be fixed and done with because driving an hour and 15 mins to the dealer each week is getting kind of old.

So the car went in for the second time last week. Dealer held on to it for a couple of days because "he had to order a part." I got there to pick it up this weekend and he said they replaced the same sensor that they originally placed two weeks ago: "We must have put a bad sensor in." I think it's complete B.S. but the light was off. Drove it an hour home, had dinner, went out and on the way home my favorite check engine light came back on. It was another P0153 code. Needless to say, I'm pretty ticked. The car was purchased almost a month ago from a used dealer. It's on a 90 day warranty from them which is why I haven't had to pay for any of the work done on it so far. But regardless this is getting pretty rediculous. This will be the third time that the car goes in for the same problem. Think I should look into MA Lemon Laws? I know its because the exhaust system (which was on the car when I bought it) is upsetting the computer/sensors, but I'm sick of the light. And I don't want to just turn off the sensors, I'd rather fix the problem completely. Any insight into what I should do? Basically I want to go into the dealer and tell him listen, you're clearly incapable of fixing the problem, so hear is the problem and what needs to be done.