That can be your first and ONLY indication of:
1. Bad BCS.
2. Bad BCA.
3. Vacuum leaks (you'll notice it idling at less vacuum)
4. Boost leaks.
5. Belt slip.
Make sure you select a gauge within the range you EXPECT to run. Don't get a 30psi gauge. If you don't plan to mod much beyond a 3.4", get a 10psi. If you plan to go further, get a 12-15 psi. You'll have to hunt to find the one you need. The point being you get more accurate readings if you're not watching teeny little needle movements on a 200psi import-tuner boost gauge.