I wouldn't recommend digging them up either - at least not all of them. If they are already stressed, it would be a bad idead to transplant them without solving the issue, and if you have clay soil and are adding compost regularly, you should have plenty of nutrients. Maybe move just one to another location and see if it does better there (but of course, that still wouldn't tell you why)
This is getting to be a bit of a puzzler.
Did stone's description of fungus blight sound familiar? Or are they just turning yellow in general? Is anything getting slimy at the soil surface?
Crowding might be an issue - but that seems an awfully severe reaction unless they are terribly crowded?
Maybe it's something you have been adding rather than something you haven't. Is it possible your compost has something undesirable in it, like herbicide, or excess salt? (someone mentioned recently that certain manures can be rather salty) I see you are out west, and have heard clay soils there can be overly saline, but don't know much about it; is that an issue where you are?
