My 1.8d non turbo ford escort looses power quicker than it should

My ford escort 1.8d non turbo is lacking power under load when throttle is pressed fully,if i lift the throttle pedal off a little it seems to pick up until the engine gradually picks up to match the ammount of throttle being used but at the next incline losses power again when u need full throttle which means i have to lift off throttle slightly and change down gears earlier than i would have to if i didnt have this power loss,its not a vacuum leak and ive also put in a tin of forte engine and injector cleaner thinking that the injectors may have been dirty or blocked due to the cars age,please help.