What I suggest before you spend lots of money is to obtain a vacuum elbow replacement kit and replace all the rubber vacuum elbows. I had the same particular problem and the solution was the one vacuum elbow under the intake manifold next to the water thermostat on the left (passenger) side of the car. Very difficult to get to. Also, there is a very long rubber hose also under the intake manifold that if you remove it to replace the rubber elbow, I strongly suggest you replace this hose. It gets brittle and cracks and can lead to intermittent problems.

I got so frustrated that I replaced all the rubber vacuum hoses and elbows.

One other thing I did, since I also kept getting trouble codes associated with exhaust, intake, vacuum, and fuel injectors (after replacing each one), which actually fixed most of my other problems, was I disconnected the battery, and disconnected all the electrical connectors (and I mean each one) in the engine bay. They were all severely corroded. I used electronic contact cleaner and cleaned and burnished each connector's contacts.

Guess what? that was what was causing the intermittent problems with my fuel injectors causing a valve error code.

All new rubber hoses and elbows and clean and burnish all connectors and the car runs excellent now.