boost problem
 

I have a 1993 RX7. Anyways my car does kick ass and boosts 13psi most of the time. The problem i am having sometimes is that when i shift my boost drops to like 1-2 psi and will not go any higher. But if i let go of the gas pedal and then slam on the gas pedal again i get my 13psi back. Also when i do get that 1-2 i can hear my turbos winding up. Anybody got any ideas what this might be. Thanks.