Look this is going to sound funny to you but if it were me I would check the air cleaner to be sure it is flowing air properly and if in doubt get a new one. A clogged air cleaner will cause the car to idle up also there are some sensors on the air intake that do malfunction and may not "throw a code" so checking them is prudent as well.
Failing all of this I suggest you call D3 performance (562) 216-8919. http://www.d3cadillac.com
Tell them I sent you. They are on top of all of the idle problem anomolies because the they do performance kits for the XLR and the XLR-V. They may have some ideas that will resolve your issue.
Please keep us informed.
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