Z3 Problem Solved!
Finally, after months of searching, reading, mulling and question asking, my Z's idle issues are fixed.
For quite some time the car knocked occasionally at idle and would either set lower than 500 or greater than 1200 rpm.
The recipe: a brand new Idle Regulation Valve.
Got it in the mail yesterday, installed it, and now she purrs like a kitten.
I only post this because 90 percent of the knowledge gained in this situation came from the forums.
Thanks all that have helped!