Hiya Pete, scanner should show closed throttle at idle on live data, if not will show symptoms that you have.There is an idle position switch inside the air valve spindle ( I know because I took one apart to try to repair it! i didn't succeed!!). The switch informs the ECU that the throttle is closed and the ECU then goes to the "idle map" for ignition timing and mixture control. If the ECU does not see a closed throttle signal, it will assume an over run fuel cutoff situation and intermittently shut off the injectors.
If the valve was renewed recently, I sympathise with you,as obviously it's reasonable to assume that it wouldn't be faulty. Where did it come from? China, maybe? Regards, Gwyn