Depending on the quality and length of vacuum hose being used, you could be setting up some "echos" or "harmonic" pulses in those extremely long, and probably soft vacuum lines going into the in cocpit valve!
Try this, disconnect those long hoses and move the valve into the engine bay with much shorter lines (somewhere closer to the turbo).
This will likely clear up your problem(assuming the vacuum lines are very soft)
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TurboDave & Kathy Huinker,
mailto:TurboDave@turbobuicks.comTurboDave@turbobuicks.com</A>
TurboD Racing,BGNRA San Diego,GSCA-5311,Turbo Buick Web Ring
San Diego CA
http://home.san.rr.com/buickv6/ 86 GN 11.50; 118.50 "corrected"(actually 11.80@115.3 at 2,900 feet).
(MM/MAF Xlator/Extender,MM BstC Boost Command,THDP,PTE44,CAS V2FM,MSD50's,9.5"vigilante 3500)
87 GN 12.95@105.50(Stock)
84 L69 Z28