[ggoose]

If I read this correctly, replacing the transmission and torque converter did not resolve the issue? This points away from the transmission as I would not expect 2 transmissions and torque converters to have the same issue.

There may be diagnostic codes active or stored that do not light the check engine lamp. You would need a high end scanner to pull these codes.

One area that comes to mind is the IAC (Idle Air Control). It is responsible to adjust the engine speed when the throttle is not open. If it is sticking it would not be able to adjust the incoming air quick enough to prevent stalling. For example, when you shift into drive, the IAC will open to allow more air in to increase the engine rpm to compensate for the added load from the transmission. When you come to a stop, the throttle is closed completely and the IAC would open to add air and keep the engine rpms up. That it seems to occur after 20 minutes of driving indicates it may be affected by the heat in the engine compartment.