The ATF heat exchanger in the radiator is leaking. You will end up with coolant contamination of the transmission as it gets worse. With the engine running the mean pressure of the trans oil is higher that the cooling system pressure. But, when the pump in the trans is not producing pressure and the engine is still hot from recent running, the pressure gradient will change and you will get coolant entering the ATF cooling loop. Replace the radiator ASAP. You do not want water or ethylene glycol in your transmission..
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.