Just in case it is a sporadic hardware issue:
I had a similar problem and downloaded a few codecs, but found that going to Tools-->Options-->Performance and just reducing the video acceleration did the most help for one of the episodes of 'Peach Girl'. (Can't remember now whether it was the first or second ep, sorry.)
I re-set it back when I finish watching, because it works fine for the other episode and for the anime and things I watch on here. It just seems to act up sometimes. ^^;
Hope you managed to fix it!
(I did post this back in the 'Peach Girl' thread under 'Current/Future Projects', sorry if you already saw it there!)