2003 Ford Explorer - why does my car feel like it's going to lose control?

I know it's a weird question but I've had my car looked at 3 times and gotten all new ball joints and new brakes and rotors.
When I'm driving (usually on the highway or above 40mph) it will sometimes feel like my car gets stuck. It's really hard to explain what the 'stuck' feeling is. It doesn't stop or anything like that - but the wheel will get really tight and hold for a few seconds. When it releases by car will swing a little - enough to scare me into thinking I'm going to lose control of it. Someone told me it might be my sway bar? But I honestly don't know what's wrong with it. Any advice will help!

Your right about explaining "stuck". Just a guess is all I can do. When this stuck happens are you by chance off the throttle? Like in slowing down. I'm thinking your engine is trying to stall and the RPM goes low to a point the power steering pump can't pump enough to assist. Like I said just a guess. I base this on the fact it is a Ford which have trouble with dirty throttle body and clogged air by pass valves causing cars to stall at red lights or stop signs.