It's and electronic tranny. It takes it's shift cues from the shifter position switch on the side of the tranny. Your shift lever is out of adjustment with the switch. Follow the above listed procedure to fix it.

This tranny applies converter lockup in 3rd and 4th, so you'll feel a "shift" as it locks up - the engine will drop 2-300rpms as the converter clutch locks up.

I am opposed to power flushing - it stirs up gunk better left alone - I take loose a cooler line and let it drain to waste and add fluid at the filler neck until it runs clean out the drain. Engine must be running, of course, to run the tranny pump...