One of my co-workers works in his dad's shop on evenings and drag races on the weekends. We were chatting about this just a few weeks ago. He said that it's not uncommon for all the problems above to surface after a high-mileage trans that hasn't ben flushed suddenly has fresh fluid in it. He attributed it to the detergents in the fresh fluid stripping build-ups and clogging critical passageways and suddenly un-sticking stuff that should stay stuck. Personally, I've always had fluid and filter changes every 30k, and have yet to have a major issue with 124k on the original trans. (knocking on wood) If you've got 100K on it and it's never been changed, I would advise to not flush the entire system. At best, drain the pan and change the filter, then top-off what was lost from the pan. That'll only be 1/4 of the system capacity, so it shouldn't cause any long-term issues.
-Joe