I have a 98 LS and it runs pretty good under no load at idle, maybe a hair under 750 RPM. However, if I rev up the engine to any RPM and qucikly release the throttle, the RPM drops to about 200 RPM and barely stays running/and sometimes stalls. Now, if I factor that into driving, coming to a stop, and hitting the brakes at the same time, it stalls most of the time. I am thinking possibly vacuum related since the brake booster seems to be affecting the issue. What sensor conrols RPM recovery when the RPM is dropped quickly? I need a starting point. The motor has about 90K on it.