My daughter has a 2006 PT Cruiser that is having a overheating issue. Hoping you guys can provide some troubleshooting guidance.

After driving for approximately 30 minutes it will overheat. When it overheats, the car will shut off completely with no warning. After a couple of hours it cools down enough to start and add water. It usually loses over a gallon of water. There is no evidence of antifreeze leaking or on the ground. Currently it just has water in it as she's been adding water to drive it. She had it pressure tested today at a garage and they said it was fine. Fans checked out ok. There is no evidence of antifreeze in the oil.

Suggestions on troubleshooting it?