I also went through a similar situation and it was frustrating. But my heat gauge would also show about 3/4 hot after the car warmed up. Usually it would be showing about 1/3 on the gauge. I could drive on the highways and not overheat (well, not red line), and the temp would fluctuate from over half to almost red line. But as soon as I had to slow down etc. It would get uncomfortably close to red line. I also showed a lot of what looked like, rusty mud in my coolant system and settling in my lower radiator hose. Anyhow. After going through and replacing the thermostat and hoses and finding a leak in my radiator, flushing the system several times. Replacing the radiator. I still had the same problem, showing 3/4 hot on the gauge. Then I had the heater core backflushed. AND that was the end of that. FULL overheating .. Redline. no more short trips with the car to see how it would react. It was down to the water pump or the heater core. I decided to start with the water pump because I was also due for a timing belt.
Anyhow again. It was the blades on the water pump, they were gone.
After the new water pump was installed. (plus, the new radiator, hoses, thermostat and fresh coolant. ) It went back to working properly.