Não faço ideia, mas eu pesquisei e achei isso: link
Sorry to bring up an old thread, but as its googles top hit for "codec overrun" I thought I would, for informational sake, chime in with my solution.
I recently had this issue, lots of downloads having Codec Overruns.
After much reasearch it turned out to be caused by the SPI Firewall thats built into my router. It was throwing away bits of information it didnt like, and corrupting files. Since turning it off everything has been perfect.