cranks over slow sometimes wont start ticking in motor will start with a jump on 1991 Mazda 929

It has a new battery new alternator when going up a inclin the car loses power. The battery,starter,alternator was tasted and fine

Asked by for the 1991 Mazda 929
charge and load test battery and recheck charging system
I gotta ask, who tasted the battery starter and alternator and said it was fine?
Napa and all those parts are new
I can't understand why the car loses power going up a incline either