Well, hello, Gary,
I have just discovered your post (I haven't looked at the site for some months) and it takes me back to my experiences a year or so ago and your ERG question struck a chord with me.
In my case (2005 407 2 L HDI), at about 180k the engine would begin a process at idle/light pedal pressure of, revs hunting, where they fell about 150 RPM, then would immediately recover, then down again, etc., and this would go on for about 20 minutes. I could not get any useful info to explain this activity, but at the end of the 20 minutes all would be OK again for some weeks, whence it would happen again. Acceleration would stop the 'hunting', and selecting 'N' at lights would stop the car moving about. So in these ways I could control the annoyance factor and live with it, so to speak.
At some point it was suggested to me that "blanking off the ERG might fix the problem". This was from a "Volvo trained mobile mechanic". This hoped for fix was not effective, unfortunately, and at last I'm about to have the EGR replaced (I got one from a wrecker and have cleaned it up and my very good mechanic will fit it shortly).
In the interim I have learned a lot about this 'hunting' issue, both from the forum and from Regan Motors Service in Auburn, Melbourne. It is all related to the matter of the DPF regeneration process, which, in my case was that I needed to have more frequent runs of sufficient speed and distance to generate the heat necessary to 'burn-off' the particle filter. AND THE EGR Valve plays a role in this process, which is inhibited by it's being blanked off. So, whilst I don't expect the un-blanking of the EGR valve will fix the 'hunting issue' it will play its part and this, together with my 'driving sensitivity', will likely allow me to control it better as I now know why it is happening!
Together with a very good auto service business here in Perth (which happily is near where I live) there is now a far better understanding of this DPF process and this is something that occurs in the trucking industry, apparently, and there is a company who cuts open the DPF casing and then 'roasts' the inner filter in a high temp oven and then reassembles/rewelds the casing and that should return the filter to new/empty condition. And as my 407 is now past 200,000 Kms and going very well it should go one for another 200,000.
In the mean time I have just purchased a 2011, 3008 with the 2 L HDI engine and at 80,000 Kms I will now research what the DPF treatments are for it...................wish me luck!
Wordy, but I hope this helps a bit.
Regards,
Notrog