Yeah, the airflow sensors have been known to cause some strange problems on the older MB's. It sounds like you did a nice job diagnosing the problem. FYI, the 70% reading with the key on indicates the vehicle has on-board diagnostic capability. 1988 was the first year for this, although California probably had it earlier. The 50% reading indicates the O2 sensor is not yet active (open loop). The oscillating readings @ 80% indicate a slightly lean mixture. In my experience the "sweet spot" for the 103 is 40-50%. As for your airflow sensor problem, I would exhaust all options/tricks of the trade before replacing it. If I remember correctly, it's not cheap. Good luck and keep us posted.