1997 Ford Crown Victoria steering is sticking and darting

I am the cars second owner. I have had it for 10 years. When you drive it and turn right or left, it does not straighten itself back up. It just seems to keep pulling in the direction it was turned. Also while driving on a straight road while holding the steering wheel it grabs. I mean the steering wheel will all of a sudden pulls right or left in a darting motion as if it had a mind of it's own. I have contacted Ford with my vin number and they said there was no recall concerning this problem. We have replaced the power steering pump and had it aliegned. It is still doing the same thing. Any thoughts. Any help would be greatly appreciated.