Ok so I have a 93 r32 skyline GTS-T. Rb20det motor with 132km all stock. Nothing done since I've gotten it in july . Don't know the history of it when it was in japan . Just know the timing belt and water pump has been replaced. The car has started to sputter at around 4k rpms and doesn't lose boost . And drives fine under normal driving. I've read it happens and ppl have changed the coils and plugs it goes away but would the coils put on a code ? My exhaust is still stock and has a little rust on the muffler . So my question is if either the cat, Resonator or muffler is clogged could that make it sputter also cause there is nowhere the exhaust pressure could leave ?