1991 MAZDA MPV LOSS OF POWER GOING UP HILL
When an engine won't run right under stress, like climbing a hill or under hard acceleration, the problem is frequently a clogged catalytic converter. This restricted exhaust flow prevents the engine from running faster and can even cause the engine to overheat. In a 32 year old car, you don't have an effective OBD system, so this is where I'd first look.
Maybe the fuel pump or a clogged fuel filter.
did you check the water pump?