My car loses power when going up a hill at 60Km/h or below. Above this there is no problem. What causes this?J?

This usually happens when ascending a steep incline at a speed of 60Km/h or below. If try engaging into low gears, this does not help much except in gear 2. Going up a steep hill or mountain at over 100Km/h is not problem. The car in question is a Japanese Toyota corolla with a 16 valve and EFI engine. The car is 17 years old with a mileage of 145000Km.