Well, it could be a number of things, and pictures/videos would help a lot. If you can, take a video with a digi cam similar to this one that shows how all you vac. lines are plumbed as well as your FMIC/Intake pipes. It doesn't have to be a super, up-close look at everything, but more of an overview of how they set it up. You may have had a Dipshit Previous Owner that just hacked into vac. lines like my MSP was when I bought it.
Things to check:
All of your intercooler clamps to make sure they're tight!
Vacuum lines could be old or leaking. Vac. leaks are the most common cause of idle issues.
Run some seafoam through the intake and gas tank. This will take off most carbon buildup anywhere.
Check your spark plugs condition (gap should 0.030 to 0.032) and PCV valve for clogs.
Since you have a FMIC, then you probably have the MAF sensor relocated... If it's not relocated, then relocate it
BTW, do you have a combo vac./boost guage? If so, what is your vac. reading at idle? What is you AF reading at idle?
Also, in this video I am shooting my vacuum manifold I made that is placed inline on the brake booster line. This made a good improvement in my BOV and WGA performance.
Search this forum over and over again. There is a lot of good information here. Sorry for the long read, but I think we can fix this and those dumbasses at Millenium should have addressed these issues before you left the lot.