[SoCal LS-L]

Makes sense..... the vacuum may not be sufficient when the idle is stumbling, and when the SVX is going very slow, its always right around idle RPMs. The vacuum in your booster might not be enough to keep adequate boost so that may be what youre feeling. Also the check valve which keeps vacuum applied to the booster even when the car stalls or has momentary low vacuum could be going bad allowing vacuum to leak back into the supply line from the booster.