1999 Mercury Sable Car engine spudders when it goes up a hi

I've had problems with this car in the past about overheating due to the propeller in the water pump being deteriorated and red muck circulating through the radiator as well as cracked and bad freeze plugs. So I got a new radiator, a new water pump, and some new freeze plugs to see if that was the cause and yes it stopped overheating. However, the engine would spudder just driving down the street so my dad replaced all the spark plugs and noticed they were bad and that seemed to be the temporary solution. Then when I went back up to school I notice the engine still spudders when I go up a hill of some kind and I notice some kind of steam coming from the reservoir even though it shouldn't be overheating after I put new antifreeze in.