canon 500d real autofocus issue - not newbie

Hi,

I've been shooting with the 500d for more than two years using different lenses and never had autofocus problems.

Two weeks ago after taking some sun shots I started noticing obvious autofocus issues with the simplest shots, something I realized straight away being used to this camera.

I have tested it with different lenses under different light conditions, mainly on tripod just shooting hundreds shots at different focal length, all apertures and checking each focus point.

It appears to be a random problem that I cannot replicate but it so evident, when choosing a focus point and there is no focus at all on that point on the image. eg 10 shots with the same settings, 2-3 of them are sometimes completely out of focus on the selected focus point. This doesn't seem to occur when switching to manual focus.

I am not sure what it is but I'm certain it is related to the body as it occurs with different lenses (EF, EF-S, USM, macro)

Can it be caused by some internal dirt?
Any idea?