What cause a 92 Nissan bluebird car to lose power up hill in high speed 4 or 5 gear?

Consider: it takes more energy to start moving something from rest that it is to keep something in motion once it is already moving.

The reason that a vehicle will lose power while climbing in a higher gear is the same reason that it is very difficult to move from a stop while in a high gear. Those gears are designed for keeping higher speeds constant, and, as such, are not designed for increasing power at the wheels. Lower gears are smaller in order to provide more power to the wheels when it's needed most.