Hi Andy
Disclaimer: I don't use Windigipet but Rocrail, so please take my opinion with a grain of salt.

Could the SDS loco ignore orders?: I use quite a number of those locos (10 of them) and NEVER has one of them ignored orders but of course a decoder may die progressively (with or without SDS motors)

What causes the loco goes past blocks w/o slowing down and stops after 1 to 2 meters? One possibility is the system (Windigipet + Ecos + S88) is overwhelmed by an avalanche of info. This happens when a bad contact changes often each time creating an information and loading the system (the information queue).
If you observe this only with this loco (and no other), maybe the decoder needs replacing (mLD3 or mSD3) with a proper motor type declaration (SDS)
Here is the manual https://static.maerklin....bb60f4f6131663855985.pdf
See page 27 and following
For SDS motors:
CV52=1
CV56=0
CV51=24 or 0 (depending if AUX3 and AUX4 have an amplified output.
AUX3 and AUX4 must be reserved to the SDS motor and mapped as active both for stop (S) and (F) (Fahrt=run)

Since this can be tricky without a CS3 or CS2 (you have an ECOS), if you have another Märklin SDS loco with a 21MTC interface decoder, you could swap the other loco's decoder with the 39837 one.

I hope this can help
Cheers
Jean

I had a similar experience with a SBB Re 460, the Märklin 39461. It’s a newer engine (and decoder?) than yours, but still it was a firmware issue (ref. to posts #293-300):

https://www.marklin-user...he-SBB-Giruno#post655726