Its fly by wire. Electrical signal makes the motor in the throttle open so if its sticking it may be the cables or linkages sticking for the electrical reading but may be the actuator itself. Remember engine bay harnesses are prone heat and cracking and can "short" causing all types of issues and if the ETA you bought was a shelf model then the inside of it may have been on the shelf a long time and the wires inside of it will fail just like the harness being of the inferior material.

I have a 93 400E and have replaced the upper harness, I plan on redoing the lower harness on my own when i get a chance and will take my ETA out at the same time and put liquid insullation on the wires to hold them apart and insulated. People usually completly rewire the unit but I used the idea with the oil sending unit and after 4 years its been good to go so I am gonna try. I cant make it any worse.