Hey Subaru7291,
It would be very helpful to know the mileage on your car. From your description, it sounds like a bad coolant temperature sensor.
This sensor tells the computer how much gas to put out as soon as you turn the car on. Was your "local dealer" a Subaru dealer?
The best way to pinpoint this problem is to pull codes from your car's computer.
If the MAF is bad, usually the car will run for 5 seconds and shutoff. However, it is a good idea to clean the connections.
The o2 sensor has a direct relation to the gas mixture so that is a likely possibility. So to discern between the coolant temperature sensor and the o2 sensor, you must note the idle when you turn the car on while it is cold first thing in the morning. Does your RPM needle jump up and down until the car warms up? Is it necessary to idle the car with your foot on the pedal until it warms up? If so, these are indications of a bad coolant temperature sensor; sometimes you don't get codes for this.
Does your "check engine" light come on and stay on?
As well, you have a throttle position sensor which could be bad or out of adjustment.
The coolant temperature sensor is to the left and just behind the alternator. The wires for this sensor can be frayed or corroded. Using a small screwdriver, remove the clip from this sensor; unplug the wires and look for green corrosion. Clean it out or replace the sensor. Note: If you XT has more than 130,000 miles on the coolant temperature sensor is past it's prime. What can happen is that your plugs are fouling while you warm up and then you run poorly from that point.
This is guesswork without seeing your car. A Subaru dealer has the diagnostic tools to tells you which sensors are bad if any or whether your front four injectors are bad.
One further issue: the o2 sensor and the oxygen sensor are one and the same.
Good luck.
Mark in Connecticut.