Today I had to go out, and I made arrangements in case I ended up on the side of the road somewhere. I drove about 20 miles, and only about 4 miles out did the gauge think about going more than halfway. But when I parked, I thought I heard gurgling/boiling. I drove the 20 miles home and the same thing happened, with the sound and the needle.
I didn't feel a great deal of heat from the engine, but when I opened the hood, the reservoir was boiling and steam was coming out around the cap. So I closed the hood
I don't want to try driving it to town but am going to find someone else that can pick me up a new cap and we'll try that.