My "93" bonneville heated up and wouldn't move-coolant hot & had trans. fluid in it. Trans. fluid level was low.
Can transmission fluid get into coolant? on 1993 Pontiac Bonneville
Yes, your radiator has a transmission cooler built into it. If the transmission cooler starts to leak, it will pump transmission fluid into the coolant. If this is the case, you should see transmission fluid floating in your cooling system, either in the overflow jug or under the radiator cap.
It is possible depending on the setup of your radiator, I'm not too familiar with pontiacs, but its possible that you have a cracked radiator.