I recently bought a used 2005 Focus ZX4 SE (manual transmission) with about 53000 miles, so I don't know the maintenance history.
On the highway, my temperature gauge will sometimes start creeping up from dead center towards the high end. It tends to happen if I'm going about 70 mph, with the engine around 3,000 RPM. It doesn't seem to happen if I'm driving slower in a lower gear but still at 3,000 RPM, but admittedly I don't do this very often so I can't say for sure it wouldn't happen. It also only happens if I don't have the fan on high or temperature set to the highest.
Once it does start creeping up, it will sometimes sort itself out and go back down after reaching about 2/3 of the way to redline with no intervention on my part.
Usually, though, the temperature keeps climbing. When it gets close to redline, I can turn the heat all the way up and the fan on high and the temperature pretty quickly returns to normal.
I brought it to a dealer, but because the problem is intermittent, they weren't able to reproduce the problem so didn't have much to report other than coolant levels are normal and both the engine side fans are working properly and to bring it back in if it keeps happening.
Anybody got ideas about what this could possibly be?
Additional information: the heater works great and doesn't ever blow cold air. In fact, it gets so hot with the heaters on high when it does start to overheat that I have to crack the windows in the car, even in the cold MN winter. I also have noticed that the engine seems to get up to temperature very quickly - much quicker than any previous car I've owned.