What file extension, format and decoder (DirectShow Filter)?
eg: .MPG | MPEG-2 | Elecard MPEG-2 Decoder
You can find out what DShow Filter is being used via File Info (Alt+3)
There's been a few other reports of this happening, but we weren't able to preproduce the problem. At a guess, the problem possibly lies with an old/funky/crappy decoder.
Try using ffdshow instead.
More info here: