2006 2.5L overheating

My wife's Beetle with roughly 25k miles has indicated an overheat twice in the last two weeks. Both times it occurred in stop and go traffic in the Texas afternoon heat, and both times it stopped after she turned the heater on for a few minutes. Coolant level appears fine. She doesn't know if the fan was running at the time but it definitely was when she pulled into the driveway a little while ago. The top radiator hose is very hot, not sure about the bottom one (how do you reach it without burning the hell out of yourself?) In another overheating/water pump thread someone suggested looking for coolant flowing back into the reservoir. Can't really see through the plastic but the line of coolant that is visible from the outside appears smooth. The dealer checked it during regular maintenance but couldn't reproduce it. It drives me crazy that there is only an "idiot light" and no temp gauge to double check! :banghead: I would appreciate any insight or advice for other things to try. Thanks in advance