Sure sounds like a classic case of Crankshaft Position Sensor.
http://catalog.worldpac.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1XC0ZKQX92BW0NRV3A&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=2000&product=F6010-100664&application=000449197
No, you can not check the sensor, unless you have testing equipment WAY beyond what you would find in the toolbox of the average person.
A low octane fuel would not cause any of your described symptoms.
A little less power maybe, but not an outright stalling.

__________________
2007 C 230 Sport.