My 1999 does the same with all heating and cooling functions. I found this on a recall website, don't ask which i dont remember. What happens is when you get on the gas, the engine sucks more air from the intake, which is where your vaccum hoses for the HVAC run from. This extra pull to the engine takes away from the HVAC vaccum causing it to engage a different blower direction. It probably starts blowing out of the feet only or the defrost, thats what mine does. You can supposedly fix this by buying somekind of a bypass valve that goes on the T area up next to the firewall, however mine has it and it still does it. My truck came with it so I don't know how bad it was before this valve thing was put on but depending how how much it does it and if it only does it when you really put the petal to the metal like mine, I would leave it be instead of wasting your time and money on this valve thing.