I bought my 08 g8 v6 in October and it ended up needing timing chains so I had my uncle replace the chains with a cloyes kit I bought. A few days after getting them changed the p0420 code popped up and my car runs sluggish on the freeway. Is it possible the timing chain could be a tooth off? It has a slightly rough idle and accelerates good 1-40 mph but after 40 it acts very sluggish. It was running rich but I changed my spark plugs and injectors and it stopped the running rich issue. If the engine is cold it accelerates great too until it gets warmed up. Should I order a new cat? The car has 129,000 miles on it.