Ghosted Axis Text

I've had issues with axis 'ghosting' when saving PNG images. The code to generate plots follows:

```
plot = mlab.contour3d(scalars, contours=10, opacity=.8, transparent=True, figure=predictFig)
plot.compute_normals = False
a = mlab.axes(xlabel='Area', ylabel='Aspect Ratio', zlabel='Sweep', ranges=ranges1, nb_labels=2)
a.axes.corner_offset=0.0601
a.title_text_property.font_family = 'times'
a.label_text_property.font_family = 'times'
engine = mlab.get_engine()
scene = engine.current_scene
scene.scene.isometric_view()
f = mlab.gcf()
f.scene.camera.zoom(.9)
f.scene.render()
mlab.savefig('%s_prediction.pdf' % name, figure=predictFig)
```

This sometimes works:

And sometimes produces ghosted text:

Any ideas about what's going on?