Well I took a shot in the dark and against what everyone says not to do, I went ahead and did it: I went out and bought a new O2 sensor (the 1990 Ford Mustang 5.0). Well it wasn't a complete shot in the dark. I had the car running and disconnected the O2 sensor and the car was running the same as when the O2 sensor was connected, only difference was that I got an CEL code 22. I looked at my records and saw that almost everything was relatively new and the O2 sensor was what stood out at last being replaced almost 35k miles previously. I know that I think the O2 sensors are recommended for every 60k miles, but since I can do the work myself, it doesn't bother me to do it sooner. Changed it in the parking lot at autozone in 15 mins right after I bought it. Drove the car for about an hour in stop/go traffic and the idle was a steady 500-600 rpm. When it was idling high I was at 1200-1500 rpm. I can feel slight roughness/vibrations with the car even though the idle is steady, and my spark plugs were done about 15k miles ago, so I will replace those and hope that the vibration ceases. My coolant temp sensor was replaced on the engine block was replaced sometime in mid 2013.

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1990 190E 3.0L