Auto Parts: chevy 3.1 idle surge, cavalier z24, vacuum problem

Question
I have a 1994 cavalier z24 with a 3.1 mpfi engine,it runs fine when im driving but when im stopped at a stoplight the rpms start to surge up and down from the normal 600 down to around 400 and almost feels that it is going to stall.It has been doing this for a couple years but during the summer it doesnt happen that often. I know the egr valve is bad but it has been bad long before this problem started, I have replaced the tps sensor and cleaned the iac valve when i did the tps.But im still thinking the iac valve could still be bad, or the egr evetually started to cause problems, or I also heard that these engines were known for intake manifolds going bad, and i dont notice any vaccuum leaks. Im a member of a website thats devoted to these cars and know one on there could help so i just need a good opinion. I dont have the money to just start replacing things until i find the problem.Any help would be appreciated.

Answer
    It is almost certainly a vacuum problem.  That means it could be the idle air control, the EGR valve, or the manifold leaking.  It could even be two together, or all three.  I would get some throttle body cleaner, pull out the IAC motor, and clean the passage behind it real well.  Pull the hose off the throttle body and clean inside it and around the throttle blade.  Then put it back together with a new IAC motor (they're cheap).  If you know that the
EGR valve is bad, then replace it, too.  Use some of the throttle body cleaner on the passages behind the EGR, just in case they are dirty.  After taking care of all this, drive the car until the computer resets (you should disconnect the battery while doing these repairs).  If the idle problem is still present, and you know that all of the components are working (since they're new), then you will need to check the intake manifold.  Take a spray can of carb cleaner (you could use the leftover throttle body cleaner if you want), and spray very small amounts of it around the edge of the manifold where it attaches to the heads.  If there is a leak there, you will hear the engine speed change when you spray the area where it is leaking.  After checking all around the perimeter of the manifold, try spraying a little into the oil filler cap.  If the manifold is leaking vacuum into the crankcase, then this should result in the same engine speed change described earlier.  If none of this gets rid of the problem, then you need to get a mechanic to use a real time scanner to check everything while the engine is running.