Hi,,
First off I’ll let you know I don’t know anything about the 850. As far as that goes not much more newer than my 1993 240. 🙂

I curious though as to why or how you know it’s not something else besides a limp mode activation?
Is there a specific code that comes and goes each time or dies it just run lousy at low idling speeds.

I would wonder if it isn’t an EGR problem as they are only supposed to get activated during idling.
A EGR code may be too generic or in its temperatures isn’t swinging enough to trigger it.

A little air in the wrong places at the wrong time might appear as limp mode.

The Park and Low gear selection does throw some wrenches on the floor though?
Later automatics did get into lot of solenoid trickery.

Keep us updated.

Phil