My gently used Canon EF 85mm f1.8 has suddenly started having focussing issues.

-it focusses well up to about 4-5 m
-beyond this, it does not lock focus, and chatters between two focus points, eg Infinity<=>5m about twice a second
-manual focus is still perfect
-make no difference if its on one-shot or servo focus mode
-same behaviour on several camera bodies
-AF/Manual switch is OK

Has anybody had this issue? There is very little info available online. One article I have found suggest its the focus sensor at the front-end of the lens, but I'd like some opinions before I attempt to disassemble this lens, or pay to have it fixed.