[Charles]

Wish I could post something useful here, but the problem has not resurfaced.

I think the radiator cap is in working order, but I took a look at the reservoir opening, and the plastic seems to be worn. That would explain how easy it is to turn the cap, but it looks like it still holds pressure.

Now, I just drive with a jug of coolant and a gallon of water in the back seat.

Again, I really can't take much time to mess with the car until after grad school (which, unfortunately, is looking like a moving target these days).