shaking boost gauge needle on vacum

[96Tsi]

I just installed a boost gauge and a manual boost controller on a 2g talon. I'm running 16 pounds boost. When I'm in park i read about 18 pounds of vacuum. But when i put it in drive and hold the throttle steady any time below 2000 rps the needle shakes up and down very quickly in the vacuum side as wide as 8 pound notches until i hit 2000 Rpms and the needle goes right to zero and smooth on up boost with acceleration. Now i know the turbo is set up to kick in at 2000 rpms but why does the needle shake so radically on vacuum below 2000 rpms?

 

[Defiant]

Probably a fault in the line (kink, etc.).
http://www.iwemalpg.com/Vacuum_gauge.htm