Not that it helps MikeW, but I've had some great results following tidbits of advice I have seen him drop. I almost only use it in full sun, keep the aperture no lower than 7, and use it on my monopod with the stabilization on. Finally, I avoid going over 500mm unless I flat have too. If the above parameters can't be met then I throw it on my full frame and let the iso go where it needs to to meet the above criteria.
To date my copy has performed flawlessly on 2 x 7100 & 600.
Thanks,i hope your lens proves itself consistent,i have to wonder if its a QC problem so some get great lenses and some dont,the basic design and ability of the lens is great.