1992 Toyota PU overheat- very odd

Good Day. I have a 1992 Toyota pickup with quite low mileage and one owner (me). Two years ago I had a singular and peculiar experience. It never happened again and since then I have retained the same amount of coolant and oil without any additions, no smoke, no steam, no white foam, etc. so I would rather not explore the head gasket angle.

The day was a hundred degrees and I had been hauling a heavy load. After dropping off the load and driving about 50 miles, the temp guage began to redline on every hill at speeds over 25 mph, but would almost immediately fall back to cool temperatures at uphill speeds less than 25 mph, run cool at any horizontal or downhill speeds, and run cool when I stopped and idled. Under the hood, all looked normal including the overflow reservoir. But here is the kicker: when I turned the heater on full blast the gauge temperature would SHOOT UP, not down. This seems physically impossible. After about an hour of nursing the truck up hill after hill, the temperature problem stopped.

In the two years since the problem has never repeated, but to be fair I have been a bit afraid of taking another long road trip. Anyone who can solve this gets my nod, since I have worked on my own autos for decades and never saw the temp gauge rise when the heater is turned on full blast.

ok, u might think im crazy, but if it ever does it again, tap the gauge cluster hard, like a love tap. if that sends the gauge all the way back to cool that could mean that you have a loose gauge conection. you can do that or pull ur gauges and check to see if the temp gauge is all hooked up right.

also doesn't hurt to test your gauge and temperature sending unit. if your temp sendin unit is faulty it can make your gauge act like it is off it's meds.

the reason why i suggested to sucker punch (lightly) your dash method is because my car does that. it reads cool and sometimes for no reason shoots to way past hot, sometimes it stay there and sometimes it drops back down to cool just as randomly as it shot up. sometimes it would even read as hot as soon as started my car after it was parked all night. freaked me out once or twice untill i got mad and punched the gauge cluster, which sent my gauge to cool. next time it shot up randomly i gave the top of the instrument cluster a light (in my standards) tap and it reset the gauge, in my opinion, the connection is loose which is giving it a false reading.