Pull the electrical connector to the round device on the back of the IP. If idle drops to ~500 rpm or the engine stalls, something's wrong with the EDS, possibly the device with the knob. If not, the problem is with throttle control or internal to the IP.

That device with the knob doesn't have the range to set idle to 800 rpm.

I don't know what that device is called. Maybe idle trimming selector.

Sixto
87 300D