How can I tell if the catalytic converter is defective. Truck runs okay for 15 to 20 miles, then starts to hesitate, back fire and then it loses power and surges ahead almost bucking. Gets worse until finaly have to stop. Then I let it sit for 10 mins or so and it goes fairly well again for another 10 miles and starts all over again. I stop because the way it is bucking I feel like its going to self destruct and of course it makes a terrible noise backfiring and coughing, etc. There is also a burnt smell or sulfur smell when I start it up. Does not idle until it is well warmed up.