This isn't a performance topic, so I'm going to move this.
It also doesn't have anything to do with boost, it'* just that your gauge is the indicator that you have something odd going on.
The first thing I'll tell you is that this has alot to do with weather and temperature. It'* not uncommon at times. If it'* a consistent problem, I'd suggest you have a mild vacuum leak. You can pinpoint it by letting the car idle, and lightly spraying carb cleaner on each vacuum connection and hose. When you hear the idle change slightly, you found your leak.
Moving to 2000-2005.