2001 Ford Focus temperature gauge problem?

The temperature gauge on my '01 Focus (2.0 Zetec 4cyl, manual trans, 210K miles) recently went from "normal" as I had been driving 50-55mph for about 10 minutes to "red" in a matter of seconds as I was braking to make a turn, then returned to "normal" as I pulled over.

I immediately pulled to the shoulder and shut the engine off after making the turn. Saw no leaks, belt is fine, water pump turns freely, no noises or smells, coolant and oil levels were full, no "gunk" on oil cap. Turned the key back to the 'on' position and the temperature read "normal" less than 2 minutes after I had pulled over.

I had it towed home, and once home I checked the return hoses from the thermostat and radiator to the coolant reservoir - no coolant was flowing out of either of them with the engine running. (Not sure if this is normal or not?). Temp gauge once again fluctuated up and down. I turned the heat on for the short time I ran the engine and the heat blew consistently warm, not extremely hot and never fluctuated cold-to-hot etc.

I maintain the car well, the coolant system hadn't been opened since last Fall (to check antifreeze quality before a Minnesota winter) and with no obvious leaks I doubt it's just air trapped in the system (?) because I don't know how air would have gotten in.

The water pump has about 90K miles on it. Thermostat is about 5 years old. Temp sensor (or sensors?) original. Had car since new.

Any ideas greatly appreciated, thanks for your help!

I would check the wires to the sensor----the fact that the temp changed in a turn might suggest something is loose----if all OK put a new temp senser in it. A car won't "cool back down" in two minutes.

Sounds like you need to replace your thermostat. There should be fluid on both sides. Also did you check your fans to insure they both come on when vehicle at normal operating temperature? Not Likely but perhaps a wire connector was jared loose from a pot hole. The temp sensor connector is usually a slip on style that doesn't take much to come off. Replace thermostat tho sounds like main problem

It turned out the head temperature sensor was the cause. I replaced the thermostat first (because it was easy to do and cost just $7) but still had the problem with the gauge so I ended up having to change the head temperature sensor since my particular car has no coolant temp sensor.

The process of changing it involves removing the alternator (p.i.t.a.) and took me 4 hours in total, but it fixed the problem and saved me what I'm sure would have been a big mechanic's bill to get it out.