Vehicle stumbles when sitting at light (in drive). As I start to accelerate it breaks up a little then smooths out over 2000rpm. It sometimes stumbles when idling in park. (The stumbling feels like a cracked plug wire is arcing on the exhaust manifold, although not the problem). I got a "check engine" light indicating the upstream O2 sensor and a lean engine condition. My guesses are an injector. If so, how do I test them.I replaced the fuel filter and put injector cleaner in the tank. Problem still exists.