Hard starting when hot
 

2005 titan; Starts first time/every time when cold. Restarts just fine if done within ten minutes, runs great, good pickup, 15 MPG around town. Very hard to start after it sets longer than 15 minutes. I do have a couple of codes (0420 and 0430). I know the codes pertain to O2 sensors/ cat converters/ and possible exhaust leaks. When I try to start after 30/40 minutes, it usually starts after four or five attempts. New starter and fuel pump installed about two years ago. Can't afford a heavy repair bill right now. Any suggestions? Going to try and replace the O2 sensors myself. I haven't been able to locate an exhaust leak from the manifolds to the muffler. I appreciate any help. Thanks