2005 Ford Taurus Car problem

Yes I have a question about my car. My car is a 2005 ford taurus se. 4 door. 6 cylinder. When im accelerating or takin off in my car somethings makin my car pull when I get to the acceleration of 20 mph, then as it proceeds to drive it drives normal. When I drop the gas at a light or turn, and I start to accelerate it pulls or drags kinda like a bump when I get to 20 mph then once it does that its fine as long as im drivin over 20mph. So basicly. When it take off it pulls then bump at 20mph mark then its fine. Please tell me you know the answer to this problem! Thank You.