Canon EF 100-400mm f4.5-5.6L IS

My EF 100-400 Auto Focus is ineffective beyond ~ 200 feet. It is fast and accurate up to that point but beyond about 200 feet the AF clatters back and forth, disinclined to make a decision. It will often give focus confirmation without acquisition. Focus tone is accurate at all distances in MF.

This issue persists in all light and contrast conditions and at all focal lengths and apertures.

Hoping for some insight.