i once owned a 123 series-280e-1982 model-petrol injeection.
this had an adjustable cable between the throttle linkage and autogearbox.
it also had a vacuum hose between the inlet manifold and autogearbox
i remember adjusting the cable and also replacing the perished hose-as i was experiencing lazy,delayed shifts.
my intereventions did help the shift to improve.
i could not say for certain whether the same system applies to the diesel 123 series car.
the other obvious thing is a fluid and filter change and fluid level check
hope you find this helpful