The convertor locks at about 40 mph by hydraulic pressure. There's a transmission vacuum switch just under the air cleaner that will unlock it under certain vacuum conditions.
If this switch is leaking (which it often does), the transmission will hunt for a gear when holding car speed at about 45 mph.
The TVS has an adjuster screw inside. Pop the little cap, unscrew the adjuster, and give it a few turns of teflon tape. Note the number of turns when removing so you can put it back in the same place.
Thanks to Jim Wirt for this tip.