B/C you just had the head gasket done, I would check the air intake cross-over pipe clamp at the throttle end has been tightened . This is an often easy over-sight when putting the car back together..it requires a long screwdriver or extension from the front. If that clamp is not tight, you are getting unmetered air and a lean condition, most noticible at lower partial load .
While you are there , make sure the little Air Intake Temp sensor has been plugged back into the same cross-over pipe..[ 1/2 way between the MAF and the throttle end]
That could also be the 02 sensor code.