I suggest to experiment with different settings of video rendering under
rightclick > Options > Preferences > Video > Video rendering
E.g. try "Use overlay Mode 1" or "Use VMR-9 (renderless)"
(I don't think changing setting of "Draw subtitles to Overlay surface" under [.. > Preferences > Subtitles] would help, but you could try anyhow.)
(P.S. Are you sure you're using BSPlayer's native capacity to display subtitles or perhaps a filter or an external program like e.g. DVobsub? Read item (9) of my Help2Help-post.)