From the google research on P1619 it shows to be the map cooling, thermostat. This fault doesn't usually cause a check engine light so it was probably stored in there before and unrelated.

The thermostats on BMWs have the normal operational characteristics, like they open when hot, close when cool etc. but they also have a small capsule of wax that is heated by the DME to offer a greater level of control over the thermostat for more consistant temperatures etc. without having a full electrically controlled thermostat. When the heated wax part is no longer working it throws a fault like the one you have, but the thermostat usually still works as a regular piece, except for the few cases where they stick open, or rare cased they stick closed. Either way, the cure for that fault is to replace the thermostat.

But that is unrelated to the tach bouncing, check engine light, almost stall situation. When the check engine light came on was it steady or flashing?

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