We need to know what the IAC count is on a warmed up engine in Park and then in Drive. Should be 18-23 idling in Park, and about 24-30 in Drive, no climate controls turned on.

Then we can go on to see what condition the transmission fluid is in. Red with clutch dust, brown with clutch dust, black and burned? The smell of the fluid is important. If the IAC counts are nominal, then I wonder about the TCC solenoid sticking a bit. I have had to drive cars for about 20-30min above 50mph on the highway for this type of symptom to show up. Sometimes it is bad enough to stop the motor once, only to let the car restart with no idling syptoms. Clutch dust in the fluid usually is the cause....fluid change and filter will solve the sticking solenoid issue. It feels like someone driving a stick shift forgetting to disengage the clutch while hitting the brakes and killing the engine as a result. You symptom is milder, I think. Near-stall.