I noticed yesterday that when I focused on a white squirrel, even though it was large in the frame, it wasn't focused properly. Today, after getting the same issue & putting it down to crap English weather, it still did it when the sun came out! Independent of subject distance, metering area & drive speed (always on AF-C), Even on static objects the focus was bouncing back & forth. BUT, when I reduced the focal from 500mm to 400 & somethingmm, it locked on fine. at home I put on my 70-200mm - no problem.
It has also recorded the focal length of every photo as 290mm!!
I've checked all the screws are tight & cleaned the contacts on both lens & camera with isoprapnol on a cotton bud, but no change.
So it focuses fine at focal lengths up to 400mm, and thene beyond that somewhere approaching 500mm, it loses it.