.....
Aside from those things, in my mind, belt surface contamination is the major factor that's often overlooked. There are a ton of places near the belts where oil or coolant can get out and get onto the belt, especially if the car has had the front cover open and wasn't sealed well when replaced (cam install). It takes a frighteningly small amount of either of those things to make or break the belt's ability to grip, especially when you're already pressing your luck with the power transmission of the belt, at least that's what I found on my car.
Anyhow, that's about all I could think of as far as factors causing or aggravating slip. I'm not trying to make a blanket statement that there are no other factors in play, but addressing the above worked for me, and appeared the most obvious/relevant to me. What am I missing? If there was something else I was missing all that time, I'd like to know about it, even though it's not especially useful info for me anymore...especially since the regal has no blower belt at all at this point