"mega-codec pack":
Are "Remember settings"-boxes at the bottom of (rightclick >) Options > Preferences > Video > ...
... [FFDshow properties] (resp. [DivX4+ properties] and [DivX3.11 properties]) checked or unchecked?
And in those 20% of the movies you're playing, does BS.Player use any particular different codec(s)/filter(s) as compared to the remaining 80%?
(rightclick >) Options > Filters > Advanced
Any difference when it concerns movies already previously played by you or not?
(Trying to find a common denominator).