Great web resources really helped (here, cadillacfaq.com, siriusbackstage.com, etc). A huge improvement. As in my post at the Sirius forum, I think the culprit is the XM box in my V; not XM itself.
http://www.siriusbackstage.com/forum/showthread.php?t=107502
http://www.siriusbackstage.com/forum/showthread.php?t=107502