I'm by no means an expert and I don't know all the details of your problem but I do know the car has the ability to high idle when certain conditions are met. for example the car will increase rpm when the computer senses a high electrical discharge rate such as when too many electrical features are engaged. I believe temp can also cause the car to high idle if it is cold started on a cool day. This is just speculation however. The electrical discharge bit is in the manual and the cool day bit is from my observations as every time I cold start the car it idles at a set high rpm for a bit before returning to regular idle
........... and read the Acronyms sticky post just above this one - matter of fact, read every sticky in every forum you visit. Amazing how much info is posted in stickys - so you (theoretically) see that info first.
Have you studied the Cadillac Technical Archive up in the top black bar ?? Basic info for every Northstar owner.