When first started runs fine but within 10 minutes starts running really poorly. It lopes, misses and will barely even move. I have replaced the head gasket, intake manifold gasket, ecm, coolent temp sensor, and injectors with no luck. I took it to a dealership and they weren't very sure but think it has something to do with the exhaust. The cat has been gutted. Is that the problem? please any help would be great!