1997 Ford Explorer Random/hard steering problem

Steering problem
1997 Ford Explorer 6 cyl Four Wheel Drive Automatic

Hi, unfortunately I'm back with another problem. My 97 Explorer suddenly started having some steering problems today. It was fine the last time I drove it (last Thursday). Today (Sunday) I noticed when I turn the steering wheel it is hard for a second then it gets easy and then difficult to turn again.

It has spots in the steering where it gets extremely hard to turn. It happens in both directions, whether I'm turning left or right.

As I said, it was fine the last I drove it. I checked the fluid and it isn't at it's max level but it is above the minimum level. The belt seems tight enough, so I don't think it's slipping.

Could this be the power steering pump? I know when the pump goes out a car is really difficult/hard to turn, but I've never had a case where it felt like the steering wheel was hard/easy/hard/easy all while turning in one direction.

I have to hang on to the wheel with two hands at all times. There is no turning with one hand right now because while the turn may start off easy, it will suddenly stiffen up for a couple of seconds causing the wheel to just slide through my hand.

Any thoughts on this would be appreciated. Another opinion from a more experienced person would be helpful before I go and start replacing parts.

Thanks!