I have a 1999 f-350 that seems to lose power when I am accelerating from a stop and have the a/c or heat on. It does not seem to respond when I push on the gas, unless I am very lightly pushing on the pedal. I have to turn off the heat/air to get it to move. Does anyone have any suggestions? Thank you.

it has a cut off switch for the a/c pump for when you are under load it releases the clutch for full power