1994 Chevy Corsica Shakes while driving and putting a load

Very new fuel filter and air filter, checked plugs on the front three and they look pretty clean and gapped. When we go up slope and when we are at highway speeds higher than about 45mph or so it shudders. The shifting seems ok as it passes through speeds and down shift when stomping on the gas works fine too and we get good acceleration, however at normal speeds around 65 or 75 when we slowly accelerate, the shuddering happens.