2000 Volkswagen Jetta Idle problems

I have a 2000 vw jetta 1.8 turbo with a few performance parts (aem cold air intake, plugs wires, cat back greddy exuahst, greddy turbo timer) my problem is that it keeps dropping idle only after it is warmed up. I was told the first time it was the alternator (i am now on alternator #4 due to bad brushes in them) it runs great after I put the new alternator on for about 2 days now it is back to idle problems. It runs fine but at idle it will drop to about 200 rpms and then jump back up it will do this till you either give it gas or it just dies. I have replaced some of the vacum lines that were brittle. I did find that this car has an idle control valve does it need to be replaced? If any of you know anything about the idle problem please help several shops can not figure it out.