Possible vacuum leak

Hello, that definitely sounds like a vacuum leak. This is a speed density engine, so it has a MAP sensor to monitor intake manifold vacuum/negative pressure. I will post a guide below about find vacuum leaks. I would recommend starting with just a spray bottle of water and spraying around the intake and you'll be able to hear the water being suck into the engine. If that doesn't work, then you can go to the guide and use a flammable spray but since your idle is so high already will be easier and safer with just water. But most likely you'll need to replace the intake gasket. Is this the 3.8- or 3.4-liter engine? I know both are very similar.
Below are the instructions for replacing the upper intake gasket on the 3.8 engine
You'll notice on the fifth diagram when reinstalling this small vacuum source valve, make sure the O-ring on it is okay. They get dry rotted and can leak as well. And be very careful if you happen to be doing this when the engine is hot, just because of the exhaust manifold will be very hot and you don't want to get any residual oil or gas on it.
The last picture is of where the fuel line comes into the fuel rail. Just wiggle this connection a little bit. I owned at 2001 Chevrolet Impala and this line was a little loose for some reason and it would leak gas only when I was driving. So, check that for leaks, just for safety. I happened to bump into it one day and it leaked fuel all over the place.
There's just a little O-ring in there and it doesn't seal that great,

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge