If this problem is happening right after you start the car after sitting overnight, it is also a common issue and happens to almost all of us. There is a fluid drain that empties the torque converter when it sits and the front pump has to refill the transmission when you first start up. Later on in life they put a check valve into the pump to prevent this, but I think is was not until around 1978ish. So, if this is the case, when you first start the car, shift into drive, neutral, drive, neutral, then drive, about a second between the changes, and it pumps the fluid where it belongs and is good until next time you let her sit overnight. The other recommendations are also good, maintenance is usually pretty lax over the years, especially if you don't know the history of the vehicle.