GrandPrix 94 with 3.4 DOHC rough idle...help?

I have problem with my grand prix with the 3.4 dohc. What it does is sometimes it runs good and all sudden it starts changing rpms and the idle is start being really rough even sometimes the engine stalls. I was suggested that it possible that some of my sensor are bad or there is leakage with my injection vacum system. If there is something wrong with sensors which one could it be and where is located at? I would really appreciate any advices and help. Thank you.

Look for the IAC valve on the throttle body. It's an electric stepper motor that functions similarly to a carburetors choke, varying the amount of air allowed in the engine at idle and part throttle. When they fail, it will cause a surging idle like you describe. You can remove it from the throttle body and clean the carbon off of it and from the bore it rides in. The other known issue on your engine is an intake manifold leak. As the intake gasket starts to go, it will only leak after the engine is warmed up and the metal parts have expanded. You can test it by carefully spraying wd-40 or carb cleaner around the seam between the intake and the cylinder heads with the engine running. If the engine reacts as you spray a certain area, you found your vacuum leak.

Thanks a lot, will do as you suggested me and after I check all those I will reply what happened. Thanks again