You could always try manual focus
TBH that's what I do with mine more often than not.
It would be a shame not to use it because of the AF issues - many, if not all, telephoto macro lenses suffer from hunting and relatively slow focusing - because they have such a long focal range. The SP 90 is a brilliant lens (AF aside), sharp as a razor and with brilliant bokeh. It's my prefered portrait lens - well, that and the 50mm f1.4