i use k-lite mega codec pack, plus qtlite (because the monopolising apple b'stards won't allow it to be incorporated) and latest wmp codecs (extracted from wmp installs) in order to play everything in the supplied copy of media player classic; once you've ticked and tweaked a few options to let it take complete control of all supported formats
i just tested a poor quality flv for you and, via the ffdshow options that show in the taskbar when it is being used, i was able to smooth it out a bit by ticking the "post-processing" option. however, there are also more options where you choose what method mpc uses to render its output. the result was ok though, considering it was a low-res file downloaded off a french video site