Boost Gauge issues..

[Swifty1638]

I was out driving home tonight, and my boost gauge was acting funny. It's a faze, mechanical. It would spike to 30 vacuum, and then settle at 20-22 vacuum pressure. Under boost it would work, but then when I shut the car off, it's at a stationary 4-5 vacuum. The gauge has never been off on me before..how could this be? I know that I have a rather large boost leak off of the 1g bov flange, but still..could that really throw it all off?

-A. Swift