What engine does your Premio have? Have you tried to consult someone about checking for any diagnostic codes with the Toyota Intelligent Tester 2? I would definitely check your EGR Valve and make sure to get it cleaned out. I would also try to clean the EGR Passages as well, and see if that makes a difference. Yes, a Catalytic Converter contains precious metals like Platinum and Palladium, but honestly as a Chemist myself, if you really want to flush it out, I would try something less reactive than acids.