Torque Converter or Bad Trans

More details, when does it do this? all the time or just at first engage? would help.
Torque converters can leak down while sitting extended periods. Mine does. I first shift to neutral which runs the front pump and fills the converter, then at engage forward or reverse, it moves, drives fine.
Band adjust isn't so difficult. One adjuster is outside the case. The other adjuster is inside so "filter service with band adjust" is more accurate. That service with or without a bottle of additive could buy some time, even with a well aged/worn trans.
The ATP AT-205 is not transmission specific. Add it to any oil to stop leaks. It is watery like brake fluid. Label states 1 bottle per 6 quarts so 2 bottles in a torqueflght. If I was going to try a additive, CRC or Lucas.