Hmm...

Upon further research, I disconnected the coolant temperature sensor's wire harness and took it out. On this harness, there is a black wire, a yellow wire, and a green wire. The harness itself was worn out; the protective tubing had mostly worn off, and what was left crumbled as I touched it. The green wire had a spot where the green sheath had worn off, exposing bare wire. This was probably rubbing against the top of the engine, under the plastic cover. I put some electrical tape around it as a temporary fix (it'll probably melt if left on long enough), and plugged it back in.

When I started the engine, both fans came on, then went off after a couple seconds. I sat there and let the engine warm up. The temp gauge crept up--slowly--between the 100 mark and the 210 mark---around 160 maybe? I felt the upper radiator hose, and it was almost too hot to touch. The lower radiator hose was slightly cooler, but still very warm. So the radiator may be all right.

I drove the car to the end of the road (maybe 3 or 4 miles) and came back. The temp gauge didn't move much, probably because the sun had gone down, and it was cool outside. I got back home, popped the hood, and let the car idle for a few minutes. The radiator fans--both of them--suddenly turned on. I looked at the temp gauge, and it was just above the 210 mark. I revved the engine to maybe 1500 rpm and held it there, and the temp gauge almost immediately went back down to just below 210. A few seconds after that, both fans turned off. I let the car idle, the temp went back up, and the fans turned back on. Even with both fans on, the temp was getting to maybe 235 or so--around the 3/4 mark. Both radiator hoses were hot to the touch. I revved the engine, and the temp went back down to just above 210. I then turned the engine off.

So, the CTS harness probably needs to be replaced. The engine runs hot when idle, but quickly cools back to close to normal temperature when revved up. Both radiator hoses are hot when the engine is warmed up. Both fans come on when the coolant reaches a certain temperature (shouldn't one fan come on, then the other if the temp gets hotter?). The temp gauge appears to be more or less accurate.

Ideas?

EDIT: Another question: Where can I get the CTS wire harness; what part number?