I should have added..with lights, I've now gotten the habit of deleting the lights prior to assigning any additional light set... there have been just too many hangs when I've changed light sets. You can do this with a delete light python script..i think it was here in the free stuff..can't remember exactly where it came from now.

Humankind has not woven the web of life. We are but one thread within it.
Whatever we do to the web, we do to ourselves. All things are bound together.
All things connect......Chief Seattle, 1854