Peugeot 806 gasoline sometimes stands

Hi, my question is this: I've had this 806 for a few weeks now and I'm still figuring it out. Sometimes it runs perfectly, but just letting off the throttle to shift gears causes the engine to stall. As an extra comment, the speedometer isn't working properly; the needle fluctuates at low speeds and only stabilizes at 80 km/h or more. Could this be related, since I read that it receives low-speed information from the fuel injection system?

Thanks in advance.

Cleaning the intake manifold usually removes the dirt.

Thanks, Kikeme, for your reply. I just started it up, and everything seemed normal when it was cold, with the RPMs around 1100. After it warmed up, the RPMs dropped to 800. It didn't have the problem then. I'm going to check if the problem starts when it's at normal operating temperature.

This is all just for guidance since I'm not a mechanic, but at least I'm trying to gather some information for when I take it to a shop.

Cheers.

Possibly the incident is being caused due to a failure to send a signal to the speed sensor. When the injection module does not receive this signal correctly, slow speed is activated (the initialization of the autoadaptives depends on the speed signal of the vehicle) and the function that cuts off the two injectors in deceleration does not work correctly (To auxiliary not "engine cooling", reduce the emission level and reduce fuel consumption, the injection module must "disengage" the two injectors during the decelerations in motion and "release" the logo that the speed, which is captured by the speed sensor signal, drops below 15 km/h
.