My sons Doblo has the same problem. If he tries to rev it when standing still and switches the ignition off while revving it, then back on, it will rev higher. Once he lifts off the accelerator, it is back to a max 3000rpm.
Engine light is on on the dash, but this was on way before this developed. Recently it ran great for days, then you never know when it will develop again.
He has put second hand everything on it....egr valve and the sensors below the egr. the one on the turbo pipe to the airbox and radiator fan sensor....no joy! Does this sound like a fuel pump problem as mentioned, as nothing shows up on the diagnostics? Help please! He is at his whits end! Alan.