One piece cranks will feel either slightly over tight (lumpy) or slightly loose during adjustment. The tolerances are just not precise enough to achieve a perfect adjustment. I always adjust them to microscopically too tight. My experience is the same as yours, in short order they become slightly too loose. If I readjust them, they will again become slightly too loose in a short time. If I just leave them alone, they will remain microscopically too loose forever. They don't continue to get more and more loose.