Major handling issue

My 74 duster has a bad handling problem. When I'm going down the road the car could be handling normally then suddenly it will take off to left or right with out warning. Once it does this, it will be a handful till you get where you're going. I can barely move the steering wheel and the tires will squeal. It very unsafe to drive. I've notice that if I jack the front end off the ground and the front suspension completely unloaded , when I let it down and take for a ride it will handle normally till I get on the throttle. Then it starts acting up, Then I have to drive it 35mph to be safe. Anybody no whats going on here? I know the idler arm is barely touching the headers but I don't believe its enough to cause this problem. I've replaced the pitman arm, tie rod ends, idler arm. It doesn't appear the ball joints or bushings are bad but I'm certain.