If you still have DISA in your custom setup, that is a likely culprit with this sort of instability near idle. Even INPA will not test the physical functionality of a DISA flap, only the position of the actuator motor, which has a brittle, soft, plastic arm connected to the flap. Just a suggestion - pull that sucker out and make sure the flap is secure with no play. Anodized aluminum replacement parts I used in 2014 on my VF blown M54, still sold by this seller:
http://www.ebay.com/itm/400717692839