This lens will not operate in A-Dep (Auto depth of field) mode on my Canon 40D. I recently bought this lens second hand. It works very well in all other modes. Is this the start of a terminal problem? Advice please?
Maybe, it depends if you buy other Tamron lenses :)
A-Dep requires a lens which transmits distance information to the camera. This is needed to compute DOF in addition to aperture and focal-length. Not all lenses do that and I suspect you will rarely find this among third-party manufacturers. They may do it at some point - or already on newer lenses - but remember that each third-part AFAIK has to reverse-engineer the mount protocols and they do not get it 100% right which is why they occasionally stop working with newer cameras.
In other words, your lens is probably operating the best way it can and there is probably no reason for it to degrade any faster than another lens of the same quality.