Power Lost when going over hilly terrain

I Have A 08 Ser­ena, When Am on The Flat acceleration, Gear Change Is Fine. When­ever I Go Up An Steep Enough In­cline The Car Looses Power. no power in D or L . if i put the car in park (P) an give it gas, the rev count slowly moves up. i have to let the car cool down before i get back power. when  i have change oils, filters except gas filter. need help aspa?