my brother has 95 T100 automatic. he says it loses power/sputters when kicking it down in gears to go up a hill.

it has about 120k miles.  He just had new plugs put in - he said it ran better for a few weeks after the new plub, but the problem is back.  i have the same exact truck with a stick and i've never had a problem.  could his problem be a fuel injection issue?  maybe a sensor that decides the mixture of fuel?  if not what else could it be?  he's done the fuel cleaning additive routine.  he says once the transmission shifts back to the normal gear, everything is okay.  it's only the auto-downshift when he presses the gas when going up a hill.