Help please!?

I drive a 1997 Saturn LS and it just recently started acting up. Sometimes when I apply the brakes, the car will stall, and almost everytime that I start the car, I need to rev the engine a little first before it will stay running on it’s own. Often, when the car decellerates (sp?), the rpms go up to almost 4000. I also noticed some liquid on the outside of my engine. (It looked like oil, but there is nowhere that I saw that oil could have been leaking out.) Any idea what the problem could be?

My first guess is a possible leak in the brake booster. The brake booster uses engine vacuum to add power to the brakes. If the booster is leaking when the brake pedal is pushed, it acts like a large vacuum leak. This may stall the car at times or cause the RPMs to rise at other times. To confirm whither the brake booster is at fault is to temporarily disconnect and cap off the booster and see if the problem resolves itself. Do not drive the car when doing this, because with the brake booster disconnected, the brake pedal needs a lot more force to stop the car.