I'm intrigued by the fact that the problem got progressively worse as you rode. Something is changing.

--I'm guessing dirt was getting into the pivot points or cable guide, preventing the spring from pulling the cage back far enough. If so, maybe all it needs is lubrication and manipulation (move it back and forth several times to work in the new oil). You may have to lube every moving part of the whole system--the shifter, cable, cable guide (or housing, or whatever you have, and of course the derailleur...

--Less likely... perhaps road vibration has turned the adjusting screw?

--Even less likely... perhaps every time you upshift, the cable tension was pulling the whole derailleur down the seat tube a tiny bit, or twisting it around the seat tube a bit ? If so, put it back where it started and tighten it a little more. If this was happening, you'd have trouble upshifting, not downshifting, unless you have one of those backwards Suntour fd's.

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