Jame's answer seems to have good insight into this problem. The engine vacuum is maximum when the engine is idling and decreases when you step on the accelerator pedal causing the throttle plate to open and much more air to enter the engine. So you may have a marginal vacuum line connection or marginal vacuum actuated motor under your dash panel. A technician that knows where the vacuum motor is located could check the vacuum or simply make sure the vacuum hose is properly connected.