Hi SteveB

Underneath the air cleaner assembly are two vacuum transducers, one of them may be the source of your P0120 code. It may also be part of your surging problem. If the surge is still prevelant then you may want to consider new resonance flaps as they do get coked over time.

I would imagine you have already replaced all the plastic fuel lines at the pump and filter assemblies? I have repaired several intermittent surge conditions by replacing all the fuel lines in the engine compartment.