Agree. Front focus and back focus problem shows up best with large-aperture and longer focal length lens due to the shallow depth of field. Kit lens are not the best test kit. My favourite is actually 85/1.4 or the 135/1.8 but the 50mm are as good.
In fact many bodies can be a little FF/BF'ed but does not show up until you use a F1.4 lens. Send in the body first to check and try not to adjust your lens (just in case the lens is fine). Imagine you make your lens '****-eyed' so that you '****-eyed' body can see properly. Then next time you get another lens, you have the problem again except now you have 1 body plus 1 lens to calibrate.
The Sony technician is really nice, have patient to explain things to me and everything even tell me what are the good lens etc. :thumbsup: