2000 Ford Focus Temperature gage pops to red at high speed

Engine Cooling problem
2000 Ford Focus Four Wheel Drive Manual 80,000 miles

When driving at higher speeds, usually over 60mph, the temperature gage pops to red. It doesn't gradually get to red, its very sudden. If I slow down, it will go back to the middle, suddenly.

We have changed the thermostat and radiator. We have checked the water pump, sensor on the engine and have pressure tested it also.

Although pressure tests show everything is fine, the water in the car is not circulating properly, because from the radiator, one pipe (lower) is cold and the other pipe (higher) is hot after a drive. The cold pipe does warm a little, but there is a clear difference between the pipes, which suggest that the water is not circulating. However when diagnosed by two mechanics, the reason for water not circulating is hard to figure out.

I've come to 2 next steps:
1. Change the thermostat to one that opens at lower temp (approximately 70 degrees) - open at a lower temp might make the engine not overheat a high speeds?

2. There must be a block in the cooling system somewhere? Even though the radiator has been changed, it could be somewhere else?

PLease help.