You're absolutely correct: loose throttle cable. Apparently there was so much play in it that I could not get any power. This explains why my car has been loathe to change down: it thinks I haven't bee pressing the accelerator.

While the car is not exactly a beast, it is a bit faster!

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190