Don't flush the tranny. Just do a drain, filter change and refill. If you flush, you might make things much worse and unless you are prepared right now to rebuild or replace the tranny.
Sometimes when vehicles sit for any length of time, things can get a bit sticky. You can try a SeaFoam treatment
http://www.bonnevilleforum.com/t281662/ . Then change the oil and filter and see if things aren't any better.