HI, my 245 decided it was going to be stubborn the other day. The car once warmed/warm start, began choking then leveled out. Then it just kept choking itself to a stall. no gas smells were coming from the car. I had a chance to let it cool down then it idled alright and drove fine, then at lower rpms, choked itself to stall once warmed up. Things I've ruled out by cleaning /replacing: IAC, Throttle body, spark plugs, plug wires, rotor. That's about it. I suspect my O2 sensor right now, as it reads around a steady .5-.45 in open loop (cold) and drops voltage (no reading) intermittently for a few seconds (no set amount of time). I didn't read the O2 sensor when it was acting up as i was running to shut off the ignition to avoid detonation (as i suspect it's too lean). it is my belief that the vacuum hoses are alright as the cold idle is fine and the issue seems connected to the closed loop (warm). excuse me if i have open and closed loop mode confused.

So my ultimate question, is the O2 the perfect suspect or should I look at the airbox thermostat/egr valve, and vacuum lines?