Where do I begin diagnostics? on 1992 Ford Taurus

failure to shift into overdrive, then other gears. Once the car downshifts, it just tacs out and I have to put in park and turn off. When I turn it back on if the transmission is cold it works more often, as it heats up it gets harder and harder shifting. (automatic stationwagon)

1 answer
seek a free diag and est from AAmco and then shop prices