When you figure it out, will you post the result? Mine did the same thing last weekend. When it had cooled off, I popped the cap to add some coolant mix, then popped it back on. Since then, the symptom has not reappeared.
I actually saw your post while searching for posts to find out how to check to see if the auxiliary engine fan is working. My car starts to run hot in slow traffic or stop and go traffic.
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
2003 Chevrolet Suburban