Partner 1.8 Petrol Speed spins, workshop doesn’t know what to do…

Hello everyone, I have been having a problem with my Peugeot partner for a long time, 1.8 liters of petrol, 90hp, year of construction 99. After you switch off the speed does not fall off, but “remains stuck”. If you are just accelerating, it rises even further, then either stays constantly high or fluctuates something or falls slowly again. That’s different, so it doesn’t get boring… Sometimes there are also phases, because the engine runs constantly on 4000 revolutions. The following was done: after I did a little research myself, I installed another actuator/empty controller, but that didn’t bring any success. So to Peugeot: the harness was measured, no error was found. They then meant that both my old and the newly built motor were defective. So built-in idle controller returned as defective, others installed, the same again. To the highway, it wasn’t on the highway, but on red traffic lights it was a bit embarrassing. The complete throttle valve including the actuator was inserted from a scrap vehicle, again without success. So it does not seem to lie there. The workshop of my trust said that it could also be on the control unit. However, this is not certain and an exchange of the control unit could only pass through Peugeot because of the immobilizer. I myself have still noticed: the high speeds are obviously triggered by the actuator. In a phase when the rotational speed ahl actually dropped to about 1000 cc. I pulled off the plug and went on almost without a controller. Look, the speed was always lowered as fast as it should be. Only the Peugeot runs very modestly when it is cold. Apparently the actuator is incorrectly controlled. Did anyone have a similar problem? Are there other causes of error that could fit? Can the error be circled in any way? I would be very grateful for some tips.