Hello, I have a 2004 nissan sentra 1.6L, manual trans with 130K, good shape. Recently it has started to buck occasionally, usually in 2nd or 3rd gear. This became worse yesterday. Plugs are new (months), I checked them and the wires and seem OK. Rare hard starts recently, but fine today. Engine will drive but randomly rev up to 2500 or so and start to misfire. Exhaust looks and smells OK. Wondering if this could be an O2 sensor, throttle body or computer problem. Thanks!