Ford Brakes/Suspension :: Power Steering Locking Up After New Pump Install?

AC compressor clutch bearings went bad (went AWAY, actually) and the serp. belt snapped. New clutch went on just fine and works great, but after about 20 miles the power steering pump pulley cracked and started spinning on the shaft and the new serp belt jumped half off and ate itself.

So last night I finished putting in a new pump and pulley. Flushed out the lines with new fluid, manually bled the new pump and then followed the bleeding procedure in Haynes carefully. All looks good. Pump spins freely and runs very quiet. No noise or hesitation lock-to-lock. No leaks. Fluid level holds steady right at the half-way mark on the reservoir.

Sitting in my yard it turns smooth and very easy all the way through the whole range.

BUT, out on the street I'm getting very frequent but random lock-up where the wheel doesn't want to turn AT ALL (I'm a big guy and it's HARD to turn)! And then I'll jiggle it a bit and it's smooth as silk again. It's done it while moving fast, moving slow, and standing still. I drove about 30 miles testing it out and it seems completely random when it will lock up, but it did it about six times in half an hour. And when I got home and parked? Perfect, smooth steering.

What could be doing this? Prior to the troubles last week, steering was perfect, so I don't think the steering box is bad. (Could be, of course...).