1. How old is your thermostat?
2. Is it a genuine Nissan Item?
I would replace thermostat with a new one and also a GENUINE Nissan one. I had problems in the past with non-OEM parts.
Ask for the coolest thermostat because you are in a hot country. That is what I did.

3. Check the condition of your radiator. I would almost guarantee it is blocked due to age. If the core needs to be replaced, do yourself a favor and fit a triple core.
I think this would be the cheapest and quickest solutions. If these don't work, come back to us.