1986 Mercury Sable faulty temp reading?

Hello pros. My 1986 sable wagon temp indicator sticks in every so often at 3/4. Stays in place even when the car is shut down and cranked in the morn. And then will rise like it's overheating the next day. I replaced heat sensors and had a diagnostics test done and had the fan checked. All was clear. I don't have any water in the oil. But there is a bit of steam or vapor that comes from the exhaust. But I am, in a humid climate. Even just today I was driving it and the temp read like it was rising. I pulled over let it "cool down" for an hr or so. And the guage never reset. So I drove it home and checked again. No leaks. Nothing. Maybe it's just the guage? Just never had one guage go bad. Ever hear of this prob with the mercs?