I would think that fuel "quality" or some additive was causing that problem.

I would drain all of the fuel & start with a new tank, Sometimes the heater control won't completely power-up the heater circuir in the sensor.

Have you made sure that the heater circuit for the O2 sensor is working.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator