Sounds like two different problems. The occasional overdrive problem is most likely the overdrive relay. Usually just resoldering the board in those will take care of it. At least it is the logical and easie$t place to start.

The shifting from first problem might be a kickdown problem but since it goes away after warming up and you don't mention problems shifting from second I doubt the kickdown cable is to blame. It would not hurt to check it. I would get it to normal working temperature and do the flush as you had planned and see if that helps. If something is sticking it may take a while even after the flush to "clean up" If all else fails I would try one of the transmission flush products before I spent big bucks or loads of time chasing it.

Randy