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