Canon 28-105 USM / Autofocus problem - can it be fixed?

Happy New Years!

My Canon 28-105 3.5-4.5 II USM has suddenly started to give me problems and
I am writing this to try and figure out if it is worth getting it repaired
or if I have to write it off - your experience and knowledge will be greatly
appreciated;

When set on AF it is almost impossible for the camera (tested on two diff.
bodies) to autofocus. When pushing the release button half way, the lense
will adjust its focus a little bit and then simply stop. I can repeat this
until it is almost in focus, but after a while it 'resets' the focus and
starts all over again. It flat out refuses to AF no matter how many times it
try.

If I switch to MF it works fine, i.e. the camera will actually indicate when
it is in focus (when release is half down).

I have experimented with many different types of motives and lighting
conditions - same problem. I have checked these scenarioes with another USM
lense without any problems at all.

The 28-105 has hardly been used. It has been kept dry and clean in a camera
bag. The contact shoes (electrical) are in a very good condition. The optics
are clean.

The bodies I have tested on: 50E and 300D.

Any ideas and/or advice?

TIA & best rgds
Jakob