Could be the kick-down switch (under the accelerator pedal). Try testing that switch to make sure it is not stuck "on".

Have you been changing the fluid and filter on schedule? Every 15k miles if mostly in-town driving, every 30k miles if highway driving.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC