Check Engine Light?

Maybe someone can steer me in the right direction ... I have a 2005 V-8 4.7 Grand Cherokee with 96000 miles on it ... the idle on it was fluxuating ocasionally up to around 12000 RPM & down to 5000 RPM ... I brought it into a local repair shop in town with a good reputation & they suggested trying first a BG FUEL INJECTION SERVICE they said it cleans everything out & I was told the engine light may come on after a little & to bring it in & they would reset it ... well it came on they reset it & it was back on after about 10 miles so I went back & it now they say it has a P0420 code ... they said it could be a number of different things from plugs to cats to O2 sensors to exhaust or intake leaks & on & on ... I was told it was just a coinsidence? the plugs had 26000 on them so I changed them to give it a shot, after pulling them out they were ready ... went back & had them reset it & it's back on after about 10 miles ... coinsidence? any help/suggestions would be greatly appreciated ...