FrankT,

 "I have a feeling it's something to do with the actor/object that is selected when the sim starts.  I try and make sure I have the main actor and the prop in question selected before I start the sim."

On looking into my simulation/scene I had the dynamic stockings from svdl's office set conformed to the "Garter Belt" which is a character/hybrid dybamic cloth - so this might be the cause as you remark.