1997 Suburban with a long standing problem, started having the occasional misfiring or "bogging" down, kind of a lurching effect, happened at different times and with no consistent factors, sometimes at idle, sometimes at highway speed, etc... Mechanic unsure about cause of problem, pulled codes and got error messages for the O2 sensors, replaced those, still not fixed, pulled more codes got a bad catalytic converter (bank 1?) code. My mechanic doubts the code because it behaves more like an EGR valve issue, without replacing random parts on the emmissions system, any ideas of a checklist of things to work through?