Do not flush, do not flush, do not flush, do not flush (my opinion). Change the fluid and filter only. If the PO did no transmission work, all that flushing will do is remove all of that wonderful metallic/gear buildup that has been keeping the tranny from slipping. Its akin to the issues of using synthetic motor oil after a car has been run on normal oil for 100K miles, the synthetic does its job and removes gunk from the engine, but the gunk has been keeping the seals tight.

Try running with Power Mode enables, and see how that effects the shifting.