4AGE 16V engine overheat when at high speed

[chenfon]

Dear All, like to get advise for my 4AGE 16V. nowadays mine unit easily get overheat especially when i run at highway (start over heat within 5 minuite). but if run at normal speed 70-80kmh the temperature remain half (as per temp gauge).

1st i suspect the gasket issue but check radiator fluid condition is clean, no milky.
2nd suspect thermostat jam, dismantle check result OK.
3rd suspect cooling system piping scale, all check and replace new piping but result remain.
4th suspect water pump efficiency drop, dismantle the pump check condition OK (had replace new in 4 years back)
5th susepct radiator cap spring pressure drop and cause air into system, replace new result still the same.

did any advise for me to check futher? tqvm