honestly I cannot quantify what fixed it. Here's what I did.

removed both heads, had them cleaned/machined.
new head set, new head gaskets
replaced all seals on the intake and all 48 valve stem seals
replaced a bad mass air flow sensor and the harness going to it
replaced a bad oxygen sensor
forced the throttle actuators to re-learn their positions.

One or all of the above cured the problem, but whew! ...the M120 is serious business, not for the meek or faint of heart.

After this, my 6.3 seems like childs play.