It is my opinion that the original throttle assy's were less durable than what is being sold now. I did a number check and there is two possible throttle assy's. One for cars with ASR and one for cars without.
Because this device is a complicated mechanical device with many moving/wearing parts and potentiometers, I suspect that they will generally give problems. I can verify that this similar part is having problems on BMWs, VWs, Volvo's, and Saabs, having fixed and read about fixes in all these marques.
If Paul's unit was the right unit, seems that you two ought to get together and fix a car.
Bosch Master, ASE Master, L1
33 years MB technician