So this is what I think I've got:

Weak, unconditioned stifles, that have become aggravated by fly-season stamping, and further weakened by decreasing turnout on hilly pasture from 24/7 to more like 8/7 (in the name of keeping her legs dew-free). Occasionally, her stifles will lock up, which is painful, it seems. Otherwise, if the stifles do NOT lock up, she is sound and comfortable. The left side locked up more often than the right - which would coincide with the fact that she prefers the left lead, prefers to have me not post on that diagonal.

Okay. Sound unless stifles lock. If they lock, it hurts, and causes her to be cautious about the next several steps, then she walks sound again. Everything I've found about locking stifles indicates that it's a problem caused either in growing horses (under 4) or where the muscles are lacking condition.

So, cautiously, I've begun riding with an emphasis on building strength in the hindquarters, especially. She's done VERY well, in the 3 times I rode probably only locked up twice.

The first two times we walked and trotted in the yard, and worked on the hill by the driveway at the walk. The third time (yesterday) I took her across the road into the woods pasture and went "trail riding."

Stayed at a walk the entire time, going up and down gradual hills, over ditches, over logs, carefully picking paths over tree roots - all that lifting of the legs over things is supposed to be excellent for building muscle strength. She did not lock up once the entire time. Yay!

And, on a somewhat related note, as I was picking her feet, on her left hind hoof, I found a tiny hole at her inside quarter on the white line - evidence of an abscess!! Which, I believe, totally explains her extreme reactions (even though they were inconsistent) walking over gravel. Tonight I will examine it carefully and try to see what I've got - if it needs further treatment or if it's healed enough on it's own for a trim to open the hole so that dirt/manure doesn't get packed in.

I have a good, willing-to-please horse. Hope I can keep her sound!