Odd steering behavior - Any ideas/assistance?

Bear with me while I try to word this in a way that makes sense...
First, the vehicle - 1982 Mazda 626, 5 speed, RWD, A/C, power steering.
And the problem. Here's where I start having trouble trying to describe things...
Sudden steering motions, like what you might encounter if you were to look away from the road for a moment, then look back and realize you were about half a second away from going over the cliff, or smacking into a tree, or running over a nun and her class of 20 third graders and trying to go "anywhere but where I'm going" to avoid them, "hit resistance" - The wheel goes from "I could spin this thing back and forth all day using just my little finger" to "Good gawd! It's taking everything I've got to turn the wheel!"
The start of the manuever is nice and normal - Literal "fingertip control" - but once the wheel has turned something like 1/16 - 1/8 of a spin, it's almost like slamming into an extended shock absorber - it doesn't "stop dead", but the "easy turning" goes away, replaced by stiffness and a reluctance to turn that seems to take everything I've got to overcome (And I'm not exactly your typical "98 pound weakling", by any stretch - How many of those do you know that routinely toss hundred-pound bales of hay to the top of a 6-tier stack?) if I want to turn it any further.
"Normal speed" steering is unaffected - the (relatively) slow, small motions of routine steering are consistently light, easy, and precise. It's only when the wheel motion is sudden that the problem kicks in.
It seems that the problem is completely road-speed independent, occurring when I've got the car up on stands in the shop, at medium speeds, and at highway speeds. The only reasonably consistent "trigger" I can find is sudden steering wheel motion. Either direction behaves the same, although it *SEEMS* (and this may very well be my imagination, it's so slight) that it might be a tiny bit worse when trying to turn to the left.
I'm unsure whether this is a "sudden onset" thing, or if it's been going on for a while and I've only just noticed it - I don't normally have a lot of call to be making the type of "emergency manuevers" that seem to trigger the problem, but when a deer jumped out in front of me the other day on the way to work, and I tried (succesfully... Whew!) to dodge her, it was very obvious. Since then, I've been trying on and off to "duplicate" the problem so I can track it down and fix it. So far, I've been able to duplicate it, but not diagnose it.
What I've looked at as possible causes:
Power steering fluid - "Full enough". Factory book says keep it between "full" and "low". If you want me to tell you whether the dipstick is reading above or below the halfway point between "full" and "low", I'm going to have to break out measuring gear to find out. It's so close to exactly in the middle of the range that it's pretty near impossible to tell by just eyeballing it whether "close" equals "above" or "below". Fluid itself is good, with no smell or flavor of "burnt", very slightly brownish (the brand of PS juice I'm using in it comes out of the bottle with a just barely visible yellowish tinge), with no "lumps" or particles that I've been able to spot. I've followed the factory book's "bleed the PS system" procedure twice now, with no observable change in either behavior, or fluid level in the pump.
PS belt - Nearly new (Installed as PM back in June '03 when I swapped engines) in good shape, and nicely tight. PS pump runs on its own independent belt, so this isn't a case of something else slipping, grabbing, or otherwise "robbing" the PS pump of power. No signs of cracking, glazing, slipping, or any other ills.
PS pulley - Both at the crankshaft and the pump, the belt groove is straight, clean, and shows no sign of burring or other problems.
PS pump - No unusual/unexpected noises, no significant slop, runout, or eccentricity on the shaft. Fittings tight and dry, body/case in good shape, with no signs of leakage aside from the obvious "You didn't get the funnel into the filler hole, stupid" dribble.
Tie-rod ends: Just barely "sloppy" enough for me to want to be changing them out sometime in the fairly near future - As things stand now, I'd write them off as a possible problem. That could change if I'm given a reason to think otherwise.
PS unit - clean and dry, no leakage, no "play" in the boltdown, no visible problems in the connection between it and the steering wheel, pitmann arm nicely tight, with only a very tiny amount of "slop" at the extreme ends of the range.
Front-end - On stands or on the ground, still, or in motion, lock-to-lock is smooth and easy, with no "catching" or "binding", as long as the wheel is being turned "normally". Rapid wheel motion exhibits the same "I just hit a shock-absorber" feel/behavior as when the problem manifests while driving.
Tires - About 5K miles on 'em, pressure good, no sign of out-of-round, bulging, or other problems.
Alignment - I could use a "touch-up" on toe, but nothing else significantly wrong that I can tell. Car has a slight pull to the right - to the tune of "put the car in the center of the lane, center the wheel, and let go, and you'll find yourself in the ditch about 3/4 mile up the road"
I don't know what else to check...
I'm stumped. Anybody got any ideas for me?
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"Don Bruder" wrote

Could be the pump's just worn out and can't keep up with the fluid demand. Could also be restricted flow in the pump, supply hose or steering box. How's the pump pressure? (probably need special tools/gauges to check)
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