1999 Mitsubishi Eclipse shudering

I recently replaced my motor and transmission with used parts from a wrecking yard the car starts fine and idles good around 1000 rpm my problem is that when you are in gear any gear and you let the revs down to 1000 to 1500 to car starts to shudder, like the shudder you get if say you were in 4th gear doing only 10km /hour the car jerks forward even if im doing 40km/hr and I put it in 5th but once I get the revs up it shifts fine, I have to drive it hard all the time and shift at 4000 or more to shift smoothly, my mechanic said to change the mass air sensor which I believe my car is not equiped with its a gs model please help