You car should be put on the diagnostic machine. An experienced Cadillac/XLR Tech will turn up the "fault" and the code fairly quickly. Then there is a fairly sophisticated "decision tree" process to isolate the cause.
These diagnostics are printable so ask for your own personal copy.
Please let us know how this comes out for you. (BTW I am betting on a IAC module)
Dealer #2 says, no fault codes... most recent updates installed. Thinks condition is normal but has little to no experience with Vs. Basically "No Problem Found" but NOT "Can not Duplicate". I'll hook up with another local '06 V next week for a comparison and let you know.