Before spending all that time and money on the exhaust make sure the ignition is up to par. Faulty coil pack will put a lot of gremlins under the hood. Had my share of HV bugs. Then there are mystery vacuum leaks. One car I had would act up going up hill in second gear. Later found it was a vacuum line with a crack in it. It would bump the firewall when the engine torqued up and open up enough to upset the fuel system mixture.
