A couple of years ago I had a problem rather similar to what you folks seem to be describing. One of my legs started to feel just a bit uncoordinated and stiff, but without any hint of pain. The most definite symptom was that the ball of that foot would sometimes scrape the ground very slightly, not enough to make me fear stumbling and falling but just enough to be annoying. This problem appeared suddenly for no apparent reason. On a bad day I would scrape the ground about once every dozen strides, while on a good day it might be only a few times per mile. I felt a similar lack of coordination when walking fast, and it seemed somewhat better when trying to run fast than when running slowly. I scared myself reading about horrible degenerative neuromuscular diseases that have "foot drop" as a symptom, but all of those (like MS) had other symptoms that didn't match mine. I tried adding to my strength routine some leg lifting exercises, but I never quit or severely cut back on my running because there was no pain. I gave up on the idea of seeing a doctor because of the subtle nature of my problem. I just didn't think they would find anything, or at least not anything related to the problem.
After about six months of this, I entered a half-marathon. I ran a lousy time but noticed that I had only scraped my foot few times during the whole race. Since the problem was usually less bad when running hard, I didn't think too much of it; however, when I went for an easy run the following day, suddenly the problem was gone. For weeks I waited for it to return but it never did, and it has been gone now for about three years.
Perhaps the leg lifts in the weight room helped. (I still do them regularly, just in case.) Or perhaps, like so many other things, it just fixed itself.