vac leak?

don't know if this is norm or not but if i'm driving down the highway and the AC on everythings fine. if i have to push the pedal herder to pass or speed up weather it shifts to passing gear or not my air that was blowing on me will switch to blowing out the defrost vent and as soon as i let go of the pedal or resume what i was doing it will go back. floor it again and it switches again. if the trucks off and i switch it i van here the vac switch so i don't think there is a leak in any of the hoses. could it be that theres a leak some where else? maybe the intakes 2 free flowing or not enough? could it be a upper intake gasket?

Check you vacuum lines, one is pooched.