There are a couple places on this model where vacuum leaks occur, first is behind the power steering pump there is a vacuum hose that is attached to the manifold, this hose deteriorates and leaks. The other is the intake manifold gasket itself.
One way to locate a vacuum leak is to listen for a hissing sound while the engine is running. The other is to get a squirt bottle and spray water around the intake system. Use a stream of water and don't spray anything with wires, when you hit the vacuum leak, the engine will stumble a bit, which indicates you found the leak.